The "Documents" folder in the file manager is actually at /home/user/MyDocs/.documents (note the lower-case 'd').
If the package you want is in a repository, it's always* advised to add the repository rather than manually installing the package. If you do add Extras-devel, though, make sure you're aware of risks, and don't run an upgrade with it enabled, as it will probably update all sorts of things to broken versions. The reason you're getting an error while trying to install the DEB (using dpkg, I presume?) is that dpkg doesn't resolve dependencies; the package you are trying to install needs hildon-im-fkb to function, but dpkg doesn't have the capabilities to find and install it. This is one of the reasons we have repos in the first place. If you really must install things manually, then every time you see an error like that, you have to go track down the package with that name (and version, if a minimum or exact version is specified, as in this case), and install it first. It will probably give you a dozen similar errors, so repeat until it stops complaining. This is commonly known as Dependency Hell - see previous comments about the merits of repositories. :P

*okay, not "always", I suppose, but if you're doing something that requires you to not use an available repo, then you probably shouldn't be doing it if you need to ask how.