You need to install the "rootsh" package via application manager, then you can become root by typing "root" at the terminal.

Now you can install packages with "dpkg -i".

WARNING: You need to know your way around a linux system in general before doing anything as root, root has to power to do anything (including breaking the system to the point that you need to reflash).

Also note that a general ubuntu package might not work as-is (for various reasons, starting with the fundamental i386 vs armel issue), however ubuntu (or debian) package is probably very easy to recompile for Maemo5 (probably only minor modifications needed) using the SDK (aka scratchbox).