DVD Player on a Mac won't let you do screen caps. Here's a hack I've used. The first part is a one time set-up, the second part is how to do the caps.
To allow screen caps in paused DVD player:
Log in as owner or admin.
Open the Script Editor utility in ~/Applications/AppleScript/.
Type the following: do shell script "screencapture ~/Desktop/DVD-screenshot.png"
What to type. It will do the colors, don't worry about that. But, get the text perfect.
Save the Script as "DVD Screenshot" in the following location: ~/LIbrary/Application Support/DVD Player/Scripts.
If there isn't a folder called "DVD Player" in your Application Support folder, just create one. Then create a Scripts folder inside of that, and save your script there.
How to do the screencaps:
Pause the movie at the desired location, choose the script menu item "DVD Screenshot", and the entire screen (movie included) will be saved to your desktop as "DVD-Screenshot.png"
Location of the new script.
I make sure the movie controller is out of the frame before doing the capture. Also, unlike other screencaps, a new DVD screenshot will overwrite the existing one, so I edit the cap immediately.
All the screen caps in my reviews, etc. have been done this way.
-Chuck, Vegetarian fanboy