How can I ADD a drum kit in Session Drummer 2

The simplest way is to drag and drop the samples on the drum pads. The downside to that approach is that you can only use as many samples as there are pads. The more flexible approach is to create sfz files for the samples. I usually copy an existing sfz file the for the part of the kit that I want the samples to play on and edit the new file to point to the new samples. sfz files are text files that can be edited in notepad or any text editor. There are speciality editors like sfZed http://audio.clockbeat.com/sfZed.html if you prefer. Once you define the sfz files, load them into the pads and save the kit. FYI, there are more things in common between SD2 and SD3 than differences. The percussion pads are the major difference.