Please do not do anything as root unless inevitable. From a normal user account you can get root priviledges for specific actions. Thatí's much safer.
To install additional packages you have two option:
1) Go to ... --> install / uninstall applications (don't know in what menu it is and I'm not at my linux box right now) and select the packages you qwant to install. (Put the dvd / cd you used to install FC3 in the cdrom)
2) Supposing you have internet access on your linux system, use yum (or up2dat if you must) to install anything from the repo's on internet
If you want to use the second option I recommend you try yumex (gui for yum). You can find it at http://www.fedoraforum.org/
('yum extender 0.31 released')
For installing .rpm's which are already on your HD (like yumex once you download it, simply double click them. You'll get a window asking for your root password and then it'll ask you if you are sure you want to install it. After that you'll find it back in your menu and be able to use it.
Installing other packages is a little more complicated. If you download anything in tar.gz etc format (extention, you'll first have to unpack it with the right tool (archive manager will help you GUI style) and then install from the command line issuing commands like './configure', 'make' and 'make install', but lets' keep it simple for now...