1. Go to the Track menu and select Insert virtual instrument on a new track...
2. Select VSTi: UVIWorkstation (UVI) (34 out) in the dialog window and click OK
3. You will be asked to activate UVI Workstation's separate plug-ins outputs --> click No
4. Activate the Record Enable button then select a preset in UVI Workstation and enjoy!
If UVI Workstation is not listed as a plug-in in Reaper, you can read the article UVI Workstation not Showing up in my DAW.