My good sir, You press F5 and that puts a screenshot in your My Documents\My BZFlag Files\screenshots folder, that is if you are using the Windows operating system.
I was unaware that this only worked for windows. I was slightly annoyed at wHo for asking such a simple question and now I find myself doing the same thing - how do you take a screenshot under Linux then?
"Nifty News Fifty: When news breaks, we give you the pieces."
Just for reference, If you are on a iBook (and I guess a PowerBook) and you are missing the numpad, then chances are the F5 key increases the computers volume. The trick is to press the fn key (probably bottom left corner of the keyboard) AND F5.