it's pretty simple. first you have to make the kits into wav files. you can download pretty much any kind of converter for free.
next you have to find the folder in which the rest of your sounds are stored. For me it goes: My Computer-C: Drive- Program Files-Image Line- FLStudio5- Data- Patches.
Once you're in the patches folder, you'll see the rest of your sounds. you can just copy and paste your new Drum Kits into this folder. make sure FL is closed while doing this. Then when you open FL, you'll see your new shit on the left.