Trouble with screen shots from films

Last night I watched Hot Fuzz  and I made about 3 pages of notes on it. Today I have put the DVD into my laptop, ready to take screen shots for my evidence of notes, however when i've taken screen shots, this is what has saved...

It clearly shows that the DVD is open, however it's just showing a black screen with no DVD picture. I googled it to see what the problem was and I found this website:
http://oreilly.com/digitalmedia/2006/01/01/mac-os-x-screenshot-secrets.html

After looking at that, it seemed that I needed to download 'SnapNDrag', so I then went onto this website to check it out: http://www.yellowmug.com/snapndrag/ and now i've just downloaded it and I will try and suss it out and let you know how it goes!

Nightmare!