You only have to find out where it is.. :-)
Then you put yourself in /home/user, that's where you are whenever you open a new xterm, or whenever you just enter 'cd' in an xterm.
Then specify the path to where the file is, e.g. tar zxvf MyDocs/sheep.tar or MyDocs/.documents/sheep.tar (you see, I don't know exactly which path is used for mass storage, I haven't used it). You can always find it with
find . -name sheep.tar
in an xterm.