LaTeX.org

I'm sorry for sounding clueless. But how do you compile manually in a shell?

I am using an iBookG4. I have been using Texshop for almost 4 years. But I recently upgraded my OS into OS 10.5.6.

When I tried to open my documents in Texshop, I can see my documents. However, when I try to press the typeset button, I get an error message:

"Cant find required tool. /usr ...