I downloaded Easy Ubuntu and installed all the codecs but I still can't play .dat files.
'.dat' files are VCD files. Download & install the package 'totem-xine'. You can install
it using Synaptic Package Manager or 'apt-get'. Open the Terminal Windows & type:
$ sudo apt-get install totem-xine (Press Enter Key)
This will install the 'xine' plugin for Totem and you can play VCD/DVD after installation.
Totem player which comes with Ubuntu uses Gstreamer.....And I suppose u have installed Gstreamer-plugins for playing movies...
Instead use totem-xine this will play all kind of movies...
Jguru has already wrote the terminal command to do that...
After Installing it see help->about totem in the player and it will show that it is using xine to play files.