iMac Lion will not play burned dvd-r's that already have files on them, any idea how to fix this?

How did you burn these video files? Did you use iDVD, Toast, or another video encoding software?

Well, they are obviously in a format which can't be read by the computer - do they play in a regular DVD player? My Google search showed that there are too many converters to know what you used - it also showed that they can convert footage to different formats. Is it possible that you chose a format that was only compatible with a Windows machine? Since your Mac sees the DVD, you could try MPEG Streamclip:

http://www.squared5.com/svideo/mpeg-streamclip-mac.html

Install the app in your Applications folder, launch it, and drag your DVD content into the window. If it can read it, it'll play. If you want, you can then export it in a format the Mac can read to watch or burn to another DVD from there.

That recorder offers five different recording settings. Which one did you use?

Sony media are not highly regarded by most here. Most prefer Verbatim, Maxell, Fuji or Taiyo Yuden DVD-R.

I wouldn't be so quick to blame the Mac and there is no way on earth for every electronic device/product/software to be compatible with every other such product - but I have another question:

I just checked out your Protron DVR - not a lot of info I could find since it is no longer available for sale - but, it is a DVR (digital video recorder): so how did you get 8 mm tapes onto a DVD using it?

And, Klaus1 is correct: Sony and Apple do not always play nice together.

I asked previously: does your DVD play in any regular DVD player? That is an important consideration: if I produce a DVD in my DVR, I can play it on that DVR, but not anywhere else unless I have finalized the disc. Why? Because that is how DVD/DVR recorder manufacturers make their products. If I want to play my DVD anywhere but in that recorder, I have to finalize it (check your manual about that requirement for your device).

Difficult if the Mac won't recognize the disk, otherwise you could copy its contents to your desktop and convert it to some useful using mpegStreamclip.

This is why I chose to use a hard drive recorder (for time-shifting TV programs etc), which records in a format that iMovie can import, then burn to a DVD if required.

You are in Catch-22 territory!

You can't process it because your Mac won't recognize it!

Sorry we couldn't do more for you!

If it doesn't play in other players either, then it seems clear that it's the fact that you did not finalize the disk. If you no longer have that machine, there is nothing you can do about it unfortunately.

I had this issue as well.

try booting into 32bit mode.

Hold down 3 + 2 when booting up.

This worked for me.

When you reboot your machine it will go back to 64bit.