Ok, there must be a problem somewhere... Any error message when you enable it? And did you reboot? I dont think it can do it without a restart of the graphical server. Also stupid questio, but I prefer to still ask it: do you have internet access when you try to enable it? If the automatic install is not working, we can try to install the latest Nvidia driver directly from their website. It's a bit more complicated to do, but you have the latest driver plus configuration tools. There are pros and cons. Anyway if you can't install the driver fro the restricted driver manager, that could be your only solution.

Ok, I called Dell last night and resolved this. It's surprisingly simple - I was not connected to the internet when I tried to enable the driver. Unfortunately, there are no error messages and no way to know ahead of time that you need to be connected to enable this. When you are connected it goes and gets the driver automatically, but if you're not it just comes back to the window showing the drivers as not enabled and you get no error message to tell you why it didn't enable.

Interesting. To me it's a bug, I will post it in Launchpad (the bug tracking system used by Ubuntu). Hopefully they will fix this soon.