You install rootsh. What else? After installing rootsh, you open a terminal and type the word "root" without the quotation marks, and hit enter. Some people type "sudo gainroot" instead, but I'm too lazy for that. I'm just talking about how you become root in Maemo, it's all normal. You would have already had to do this to install Nitdroid to begin with, wouldn't you?

The original question was how to overclock in Maemo, not in Nitdroid, and that's the question I was answering.