If you close the Avid application, and insert the DVD into your computer, does the DVD show up on the computer? Can you access it? If not, and the previous DVD’s were read, the DVD is likely faulty. Ask them to supply another copy (they should have the originals on an internal hard drive). If the DVD shows up on the desktop, but the Avid cannot “see” the files, try copying the files to a local hard drive first, then try to import them from there. If this fails as well, double check to see if the files can be plaed back by a mediaplayer like QuickTime Player. If that fails, the files are likely corrupt.
All of this is often connected to the way that they back up their recordings to disk. Their are multiple ways, and occasionally, a DVD burn might fail.
And yes, he likely supplied you with both a (2-track) mix he made, as well as a bunch of ISO tracks (unmixed separate recording of each microphone). It depends on your preferred workflow how you need to go about that.