I had a similar problem. (See thread http://modeling.inside.quest.com/thread.jspa?threadID=4486).
I copied the files all over my hard drive and nothing worked. Once I copied the files onto the top level of my thumb drive, the files opened fine. Then I could move them back to the hard drive where they opened correctly. Once they worked, I didn’t give it too much more thought, thinking it was just me. Now that it has come up again, I downloaded “TDMReference.chm” to the root of my C: drive. It wouldn’t open. I copied it onto a floppy drive “A:” and it opened from there. When I copy the file back to my C: drive, even though I am replacing a file of the same date and time on my C: drive, now the file opens. Once you get the help files to open on an external storage device, you can copy them to whatever location you like on your hard drive and they work fine.
I have no explanation. I have a theory that it has to do with either a space character in the download path name or being on a shared device when it was first downloaded. I suppose it won’t be an issue in the future once the help is provided along with the rest of the beta release.