The "~" directory is your home directory, so that would be /home/dave or somesuch. You should have a directory in this called ".gimp" or ".gimp-2.6" (note the '.' at the beginning). Inside this directory is the "plug-ins", "scripts", etc directories where you want to put the stuff you download. The directory might not show up in your file browser unless you choose "show all files", as a '.' at the beginning makes a file or directory hidden.
The /usr/lib and /usr/share directories (and subdirectories) are only supposed to be for files that are in official packages, not for additional things that are installed later.