I have a DVD drive and I want to be able to watch movies on my computer, but every time I open xine and switch to dvd it gives the error: "The source can't be read. Maybe you don't have enough rights for this, or source doesn't contain data (e.g. not disc in drive).(Error reading from DVD.)"

I am part of the cdrom group, the disc is functional and in the computer.

Edit: Upon trying another DVD it works, but the first still doesn't. (The first being Season two of Avatar: The Last Airbender, and the second being Evan Almighty.)

Have you installed libdvdcss ?

What I would do, I would try mounting it manually as root if necessary:
If that doesn't work with the problem disk, but you can view other dvds without a problem, I would say the problem lies in the disk itself. I've found that if a disk is brand new without any smudges or scratches, my computer sometimes has trouble reading it. If I smudge the disk slightly they sometimes work. It's a crazy trick, but I don't know if it will work for you.

Neither of the solutions fixed the problem.

So it is true though that other DVDs work in xine, but this one DVD does not. I assume that you were unable to mount the disk. If you were unable to mount the disk, then no player you have installed would be able to read the disk either. If other commercial (not home-made) DVDs work, then it is not a problem with libraries or programs. Have you tried the disk in an actual DVD player?

A lot of dvds have copy protection so you do need to decrypt it, so I guess my question is did you install the CSS library?