NOTE: Omnisphere 2.5 and Keyscape are 64 bit compatible only. To revert back to Omnisphere 2.4 which supports 32 bit, install Software 2.4.2c, Patch Library 2.4.0d, and Soundsource Library 2.0.1c.
These instructions are the same on a Mac or PC, though there might be minor variations between the two platforms.
1. In Reaper, select the Insert menu and then "Virtual instrument on new track."
2. From the Virtual Instrument window, select "VSTi: Spectrasonics: Your Instrument". If you're using a Mac, you can also select the AU version.
3. Your instrument interface will appear. In the screenshot below you'll see that the Omnisphere instrument track has been created in Reaper's Mixer.
Here are some downloadable templates which you can use in Reaper: