I've installed Superior Drummer and REAPER; but when I go to click on "Insert virtual instrument on new track", Superior Drummer isn't available. Am I missing a step?
In REAPER you need to ensure that it knows where the Superior Drummer VST effect is installed. Go to Options | Preferences and in the Plug-Ins | VST section check to see if the install path for the Superior Drummer VST DLL is included in the list:
If it isn't, click Add and select the install folder for the VST DLL. Now click Re-scan.
If you still can't see it in your VSTi section, it might also be because REAPER sometimes incorrectly detects the name of Toontrack VSTs. Look for anything with "Toontrack" in the name and try that.