I've loaded plenty of .dsl files offline, so it certinly works in most cases.
I gather you're running a frugal install from your HDD. If you are running "live" from a CD, then it really should work so perhaps your .dsl is corrupt.
Can you try another .dsl as a test?
If none are working, perhaps something is wrong with your installation (assuming you are not running DSL from a CD).
Oh, also. Do you know that the package you're trying to load won't show any icons etc. once it's loaded. You need to check it's loaded by trying the "dpkg" or "apt-get" commands.