Come let's help him ehehe let's not be so picky.

Once you've unpacked the .tar.gz you can find the mplayer executable on your current directory. Either you run it as:

./mplayer <options> <movie filename to open>

(and not /.mplayer as you wrote)

or copy it to /usr/bin so that you can run it wherever you are by simply typing:

mplayer <options> <movie filename to open>

Note: if you've unpacked it onto the memory card, then you can't run it as, for security reason I suppose, the Nokia 770 won't allow any executable to be executed from the memory card. In this case either you copy it into your home directory (cp ./mplayer /home/user) or copy it into /usr/bin as stated earlier in this post of mine.