DVD will not play on PC computer.

Blame the Digital Media copyright act.

Likely reason is it was encoded to both PAL, and to a different region code (to make piracy more difficult cross borders). And your computer is blocking what it senses as something that you should not have based on location.

Try using DvdDecryptor to rip the VOB files to the hard drive. It will not work on a Mac unless the Mac has windows capability but decrypter may see the disk when Windows can't. If you can get the VOB files onto the hard drive you can use a free media reader like Mediainfo to view the details of the file. 25.000 frames per second is PAL format and 23 or 29.970(976) is NTSC. I have successfully used Nero Express to change a PAL dvd to NTSC by selecting NTSC in the options at the bottom and then when it asks near the end if I want to remain PAL I select no. Its also possible that the optical drive in your machine doesn't support the type of media (disc brand) they used and updating its firmware might correct it. The Mac's optical drive could be different. Perhaps if you used the mac to rip the VOB files out, you could transfer them over to the other maching and use Nero or another app to put them on a different type disc you know works. In Nero, if you looki in its directory for the acrtual nero.exe you will load the original Nero Burning Rom and then on the left select DVD and then in the drop down DVD-Video and then select NEW, it will create an empty VIDEO_TS folder for you to fill with the ripped VOBs.

This is caused by the DVD copy protection.
I think you can learn some knowlegde from this article
http://www.magicdvdripper.com/can-not-copy-twilight-saga-breaking-dawn-part-1-dvd.htm
In this article, explain the DVd breaking dawn why can not play and copy, all of these are due to the digital copy protection, and this article gives a solution.