1. First down the relevant plug-in files. Open Lounge™ is an easy-to-use business collaborator for RedDot CMS and is free to download.(For demonstration purposes, this tutorial will Open Lounge™ as a sample plug-in.) You can download Open Lounge™ here.
2. Place the downloaded plug-in folder - unzip or unrar it as needed - into your "/PlugIns/" folder under your RedDot CMS installation folder. It is normally here: "C:\Program Files\RedDot\CMS\ASP\PlugIns". The folder that constitutes Open Lounge™ plug-in is named "collaborator".

3. In most cases, Windows has set "Read-only" default attribute to collaborator.

4. Uncheck "Read-only" and hit "Apply"

5. Confirm that you want the change to be applied to sub-folders and files:

6. Above steps should have forced "collaborator" folder to inherit "PlugIns" folder's attributes, particularly the "security" permission settings. If for any reason your plug-in does not run later, come back to step 4. And 5. And inspect "collaborator" again and click on the "security" tab. You will have to manually add all users that are permissible to "PlugIns" folder but are denied access to "collaborator" folder. Such discrepancies are the culprit that prevents your plug-in from running in most cases - unless the plug-in itself is buggy.
7. Now it's time to log in RedDot CMS and browse to Server Manager. Expand "Administer Plug-ins" node and click on "Import Plug-in":

8. Browse to the just placed "collaborator" folder. All plug-in application settings are contained in a namesake XML file. If you happen to choose a wrong XML file in the folder, plug-in will just fail without any damage to your RedDot CMS:

9. Upon importing the proper XML file, RedDot CMS presets all plug-in settings on basis of the XML file. All you need to do at this moment is to hit the "OK" button:

10. Upon hitting the "OK" button, the plug-in is imported into RedDot CMS; however, by default, the imported plug-in is inactive. So you will need to go back to "Administer Plug-ins" and select "Edit Plug-in":

11. Once the editing screen pops up, check the "Active" box and hit "OK":

12. Now the plug-in is imported and activated. However, we are still missing one last step needed for run-time - what are the projects the newly minted plug-in is supposed to work with? To answer that question, we go back to "Administer Plug-ins" node one last time and select "Edit Project Assignments":

13. Go back to Server Manager and select the "Start" node and you will see Open Lounge™ ready to be launched. Open Lounge™ is also available via Administer Project Structure node, Project node under Administer Project Structure and the Action Menu in SmartEdit once a page is opened:

Now it's time to download Open Lounge™ and try the above-outlined steps yourself!
#RDError1
Amount of modules in license key does not correspond with the license key.
#RDError2
License key is a test license that is no longer valid.
#RDError3
Wrong license key. An error has occurred during which could not be classified.
#RDError4
License is no longer valid.
#RDError5
IP address of license key does not correspond with server IP.
#RDError6
License is not yet valid.
#RDError7
License is a cluster license. (Error message no longer valid from 6.0 on)
#RDError8
IP address of license key not valid.
#RDError9
Version of license key not valid.
#RDError10
Doubled modules in license key.
#RDError11
A module of the license is not correct.
#RDError12
License contains illegal characters.
#RDError13
Check sum is not correct.
#RDError14
Serial number of license not correct.
#RDError15
Serial number of license key does not correspond with serial number of former license key.
#RDError16
IP of loopback adapter is not supported by this license key.
#RDError17
License key does not have a valid serial number.
#RDError101
User is already logged in.
#RDError110
User does not have necessary right for executing RQL. Possible reasons: LoginGUID or session key no longer valid OR session is expired.
#RDError201
Access to database "IoAdministration" has failed.
#RDError301
Asynchronous component broken.
#RDError401
Project locked for user or user level.
#RDError405
Method not allowed.
#RDError429
ActiveX component can't create object.
#RDError510
CMS server not existing.
#RDError511
Currently updating CMS Server or CMS databases
#RDError707
LoginGUID does not correspond with user who is logged in\
#RDError800
Maximum of logins for a user has been reached.
#RDError2910
There are still references on elements of the page
#RDError2911
At least one element of the page is still a target container for a link.
#RDError3000
Package already exists.
#RDError3032
Attempt to delete Content Class although there are pages based on this Class.
#RDError3049 T
oo many users for license. Login failed. Please try again later.
#RDError4005
There is already such name for a user.
#RDError5001
Could not find path to folder OR folder does not exist anymore.
#RDError6001
File already in use in CMS.
#RDError15805
You do not have required permissions to delete page..
#RDError16997
Content Class could not be deleted because of existing pages based on this Class in other projects.