I have had this problem with my netbook. My problem has been that it doesn't install the driver (for some reason it does in the livecd mode but not when you actually install it on your computer.) What I do:
1. Stick a network cable in the computer and your internet router (if its a desktop computer and you can't move things so that you can do this, then what I'm saying won't work for you.) Wait a bit for it to automatically connect to the internet.
2. Click on "System"
3. Click on "Administration" (Or whatever the admin one is called, I'm not using Ubuntu at the moment so I forget.)
4. Click on "Update Manager"
5. Click on "Check" and enter your password if it asks. Wait for it to finish.
6. Click on "Install Updates" and enter your password if it asks. Wait for it to finish, then restart the computer and boot into Ubuntu.
7. Repeat steps 2 and 3.
8. Click on "Hardware Drivers" and wait for the little window to finish.
9. Select whichever driver you need and click "Activate" (or install, I forget what the button says.)
10. It should work after you restart the computer again