Unfortunately, drum maps in Reaper are nothing like what they are in Sonar. Basically they are just a text file with named notes. In the MIDI Editor you go to 'File/Load note names from file...'. You also have the option under the View menu of showing notes as triangles or diamonds. That's it.
Drum maps in Sonar were very robust and well designed. Reaper is more of a patch job to look a little like a drum map. A little disappointing, but still workable. It's about the only thing I miss about Sonar.
Doesn't seem to be much interest in Reaperland about drum maps. Coming from Sonar, that can be a little disconcerting if you used them a lot.
I have a little FR discussion going on about drum maps if you feel like contributing.
http://forum.cockos.com/showthread.php?t=63286
Regards,
DB