the key signature is the key that makes you sure that you are actually downloding packages from the server that should provide them and not from some other source. You can ignore it, or serch for some package with key inside its name (i don't remember the exact name) which provides the key signatures of the repository.
the previous error you get came from some dependencies problem, did you follow exactly the instructions in post #1 and #2, editing etc/apt/preferences too?