In Ubuntu 11.10 Oneiric, the original Alt+F2 run a command box is disabled by default. If you’re running on the default Unity desktop, the Dash do the Alt+F2 run command job. If you login to gnome-shell or Gnome Classic, Alt+F2 do nothing. Here’s how to enable Alt+F2 show run command prompt.

At top-right corner, click on your user account and select System Settings.

Then choose Keyboard, at Shortcuts tab click System and you will see following picture. By default “Show the run command prompt” is disabled. Double click on it and set to Alt+F2.

Also, there you can configure other useful shortcuts, such as launch email client, web browser, log out, and so on.