The first thing you must do is rename the file you want to use from whatever.rsb to armpatchXX.rsb. (XX = a number lower or equal to 20, because RSD can only handle 20 armpatches)
For example, say at the moment you have 16 armpatches in your folder and you want to add the 2ndLt.rsb armpatch, well you would rename 2ndLt.rsb to armpatch17.rsb.
Now drag your newly renamed armpatch to the Rogue Spear Data/data/armpatch folder.
Start RSD, go to options and select your armpatch.