how do I copy a DVD from my superdrive to my hard disk? (on iMac)

If the DVD plays on your computer, does it mount on your desktop? If it does mount on your desktop, as den.thed says, right -click or double click to see if it will open or play. If it shows a Video_TS folder then get Handbrake(an app) to rip it to your computer.

Insert the DVD, control (right) click on the icon that appears for the DVD and see if you have an App or Program that can open it. Then if you can open it, try using that App or Program to Import or Save the image file on the DVD.

Additionally, is that a commercial DVD or a homemade video/movie?