System Preferences, then the Layouts tab, then click the Options button (near the bottom of that window)
Click the Alt/Win key behavior drop-down, and you'll find lots of choices for changing the various modifier keys and the way they function.
If you absolutely don't see this, I am running Ubuntu 11.04 - so there may be some difference. It's a good upgrade, which you can get to by running System Administration, and open the Update Manager. You'll see the Release update at the top of that window. If you are running Ubuntu 10.04, then you will see 10.10. There's no point to running the other updates that might appear, so just click on the Upgrade button. It will take a really long time to complete - probably 2 to 3 hours. That gets you to 10.10. Running the Update Manager again, you'll see the 11.04 Release, and you can do that one then, and another 2-3 hours. When that is complete, then Update Manager will report 11.10 release. You should stop then and don't upgrade any more. The update will break the ubuntu startup kernel. I know, I have tried it 3 separate times, after looking for fix info, which I could never get to work. I think it has something to do with the OS X dual-boot, and the partition setup that needs. Anyway, I suggest that your stop at Ubuntu 11.04 - unless you can find an absolute fix for the ubuntu kernel and GUI changes, which right now prevent booting completely to 11.10 and later.
Good luck - if you find the Keyboard mods on your older Ubuntu, then you can choose not to upgrade to 11.04 if you don't really need to. It really takes a long time, (and I have a hard time being patient, _
Serendipity is a lucky guess !