It depends on the theme. Some display categories on top bar and some both categories and pages.
It is not always header.php where the call to the menubar is housed. Some theme authors tuck it in includes, classes, functions etc. and make it complicated. If you want to customize the menu it is better you read through the templates and associated files in the theme pack and do the hacking by trial and error.
For this kind of sandbox trials, it is better to install a webserver+WordPress in your system. Otherwise have a test installation of WordPress in a separate directory (folder) of your web space (call it “test bench” if you fancy!) so that your mainstream blog doesn’t get screwed up.
Yes. You can try the menubar plugins too. But again its success depends on how your theme is coded.
Cheers!
S.K