Included with Guitar Rig 5 should be a CD. Insert the CD into your computer, and run the installer. After the Guitar Rig 5 installer has completed, follow these steps to add Guitar Rig 5 to Reaper.

1. In Reaper, select Options > Preferences
2. In the Preferences dialog box, select VST
3. Select Auto-Detect

Auto-Detect should locate C:\Program Files\Native Instruments\VSTPlugins 32 bit.