Q: Adding a second monitor to a Mac Pro

I have an older Mac Pro running 10.6.8 with a 20" Cinema display, and I'd like to add an older Dell monitor. I have the Cinema display connected to the VGA port, and the Dell connected with a VGA cable and a DVI-to-VGA adapter. The Dell sees the Mac; however, I can't control it--no mouse cursor, no ability to change resolution, etc.

Can anyone walk me through this?

Thanks in advance!!