FL Studio doesn't load .sfpack or .sfark, they must be in .sf2 format. You can find converters easily enough on google, though.
As for storing them, there's a FLStudio\Data\Patches\Soundfonts\ folder, where I recommend you put them. That way they'll show up in the sample browser on the left under the Soundfonts folder.
And that brings me to:
A tip for advanced users!
This tip has mostly to do with organization and ease of navigation. As far as "where to put samples," I recommend you put all of them in a single folder like C:\Samples\, C:\ being whatever drive you want to put it on, and of course organizing the samples within that folder. This way, especially if you have multiple audio programs, there's a simple directory you can go to for everything, instead of a complicated path like C:\FLStudio6\Data\Patches\Soundfonts\Voice. I also recommend putting a \VST-DX\ folder on the same drive if you can. This is what I do, since I have FL Studio, Cool Edit, Modplug, Schism Tracker, OrgMaker, etc. Hopefully you'll find this organization tip very handy.