Ad-Hoc network in Ubuntu are a little tricky. I had to set one up myself and it took me a while to realize how silly the solution is!
The problem is, even after you declare the connection ad-hoc, the ad-hoc infrastructure is not selected by NM. (come to think of it, maybe I should file a bug against that)
The solution is to right-click on the nm-applet, select Modifying connexions (translating from french, so option name might differ a little bit here), and go to the Wireless tab.
Click on your Connection's name and click edit.
Now, make sure the Mode is set to Ad hoc and the IPv4 and IPv6 tabs methods are set to "local link only"
after this, it should work. If not, let us know!