getting access to geodistance (what do I need to install/enable) in an existing db install

Hi,
  I was running postgres 8.1 on a windows xp box and I recall
installing it with postgis option.
I had a method in a db there that called geodistance. Everything worked fine.

Now I tried to restore the db on the debian server thats running 8.1
as well. The geodistance function (or any other geometry function ) is
not available.

What am i supposed to install enable (and how) to get those
functions/casts etc to be present.
Much thanks in advance.

-assad