al3c77,
I just went through this with a Chromebook my son got for Christmas. I unstalled Ubuntu 14.04 and the KDE desktop, but for our purposes, it should be pretty much the same thing.
If, as frankbell said, support for 12.04 will run out in a few months, you may want to re-do the Crouton installation with the '-r trusty' option to get 14.04 and buy yourself a couple more years of support. I had to re-do my Crouton myself, because after using the Ubuntu for a while after installing it the first time, I realized I had picked the wrong options ('-t unity,xiwi'); this seemed to be a little too laggy for the intended purpose of the computer. I re-installed with '-t kde,extension', and things were much snappier. If you don't want to wipe the current CHROOT that you already have, you should be able to create a new, independent one by fiddling with the Crouton command-line options (-r, -t, -n, etc).
When researching this project, the HowToGeek page was one of the first resources I found, and it has good info. However, some other pages, like this one - 'LINK REMOVED UNTIL I GET MY FIRST POST >
' go a lot more in-depth into customizing and enhancing the Ubuntu part after it's installed. Specifically, if you go to the Voltron page, and scroll a little less than halfway down, to 'Step 7: Make Ubuntu Awesome', it gets into what you could be needing for your MAN issue: Installing the Ubuntu Software Center. I don't have the Chromebook with me right now to check, but I would guess that the Ubuntu MAN package is somewhere in the Software Center.
And just for info, the Chromebook in question is an Acer CB 14, 1.6 GHz Quad-core, with 4GB ram, 32GB storage. It's a very nice system, that is now running the Java version of Minecraft for my extremely happy son.
Good Luck
