Have you just noticed that your right alt key is not working in your Ubuntu?
Nope, don't be afraid, your button is ok. You just have to alter some option to make it behave like the left alt key.
1. First go to
System -> Preference -> Keyboard
2. Under Layout options tab, open the branch "third level choosers" and assign the third level chooser to any another key other than the right alt key.
It's done! :D