On Linux, it should be at ~/.bitcoin/wallet.dat.
However, this is a hidden folder. You can tell by the period at the beginning of the folder name. Ubuntu hides these for you so that your folders aren't cluttered up with application data.
So, to access it you can just press ctrl-H, which will unhide all folders. Press ctrl-H again to hide them all.
For a more permanent solution, go into Preferences and go to the Views tab. Then check "Show hidden and backup files". I wouldn't recommend that though, since you might accidentally mess with vital folders that were meant to be hidden if you leave all your hidden folders unhidden.
Hope I helped.