I haven't unzipped anything on Linux in a while. What is the extension on the file you are trying to unzip? Is it ".gz" , ".Z", ".zip" , ".tar"
Actually what is the whole name of the file you want to unzip?
Depending on the format of compression there's a few different tools you'll have to use. But HS probably already nailed it. Open up an xterm window, type in "man unzip" , hit Enter and read the man page for it. Should just be "unzip <filename>" then you may have a tarball that needs to be unarchived.