Two few things to check for a start:
Make sure you have the permission to use it. On my other system with a radeon GPU and the FLOSS driver, there was no access to the DRI for some reason, until I set up /etc/X11/xorg.conf as follows:
If you're using nouveau of the intel driver, substitute radeon for either of those. You will need to kill and restart X (including display managers). I do this by dropping to a tty and changing runlevel to 3 and back to 4 again.
I'm no expert on video but you may need to install some codecs such as the gstreamer plugins, ffmpeg, xvid, etc. You can find most of these at slackbuilds.org
Is your user in the correct groups? Post the output of