Media packs are stored in the tagged library, which is stored in a single file in the application data folder. If you have the portable version, the data folder is the "AppData" subfolder in the main application folder. If you use the installed version, the application data folder is in your user profile. The exact path depends on your Windows and user name, for example on Windows 7, it is in C:\Users\<your_user_name>\AppData\Roaming\RealWorld\RWPaint
Plug-ins are stored in a "PlugIns" subfolder in the application data folder.