if you're asking where you should put the mods that you've downloaded...just put them wherever. It doesn't matter.
Most installations of the game will include an "addons" folder that you can put all your mods into. They'll also include a "DOWNLOAD" folder, for mods that were automatically downloaded from the Master Server.
But the truth is, if you just want to load your own mods, you can just put them in the same directory as SRB2, or a subdirectory thereof. This is because, when you start up SRB2's addons menu, it defaults to the same directory that SRB2 is in. I'd recommend you use the "addons" folder though, since it's cleaner to do it that way.
All of this applies to SRB2Kart as well.