Open up /etc/apt/sources.list
, and you should see lines like the following (URLs will likely vary):
deb http://http.us.debian.org/debian stable main contrib
Simply add non-free
to the respective URLs you wish to use, i.e.:
deb http://http.us.debian.org/debian stable main contrib non-free
Running apt-get update
will update your local repo with the package listing.