Ok, Right Click > Open With > Discover is fine, but I found this to be the better.

Open a command line (Konsole). cd to where your app installer is. Then run this:

$ sudo dpkg -i your_app.deb

And that should do the trick. Note: pay attention to prompts. Sometimes you have to do a Y/N thing or just hit enter if it says something like “This will take up 800k of space or whatever.

If you see a message about missing dependencies: something like this:

dpkg: dependency problems prevent configuration of your_app:your_app depends on this_thing; however:  Package this_thing is not installed.

This command should do the trick.

$ sudo apt-get install -f

Want to uninstall?

$ sudo dpkg -r your_app

And voila.