sudo dpkg -i game.deb
In my case, I wanted to install the game I saw in Native Ubuntu Games, Epiar. So I downloaded a deb format file named epiar_0.5.0-1_i386.deb and it went on my Desktop. Browsing my desktop using Konsole, I typed
sudo dpkg -i epiar_0.5.0-1_i386.deb
and it installed okay. I was able to play the game but it is still in its infancy stage. There are a lot of things still missing like audio, an introduction, you know? game stuff! So I decided to uninstall it.
I uninstalled it by using Adept. I typed epiar on the search bar and clicked on Remove and Apply Changes.