I think the reason that it does not work is because of the directory structure of the plugins. Try doing this to get version 0.21.2 to work:
1. Delete out all the rainlendar files from your modules directory.
2. Open up your theme.rc file and set
"*NetLoadModule rainlendar-0.21.2 load rainlendar.dll"
3. Recycle. You should get a popup telling you that you need to download the rainlendar plugin. Accept and download the rainlendar plugin.
4. Next, go to your modules folder. Open up the rainlendar folder, and you will notice that all the plugin.dll files are in the root of the rainlendar directory. Create a new folder called Plugins, and place iCalPlugin, IniFormatPlugin, OutlookPlugin, and ServerPlugin in the folder. Rainlendar.dll should remain in the rainlendar-0.21.2 folder.
5. Recycle and it should work. You should also be able to update these plugins/rainlendar.dll with the ones from rc3 and have it work.