The options you have depend massively on which version of Nero you have. Or it did 5 years ago the last time I used it.

In the full version, from memory there's a menu item called "disk tools" or similar and I think it was in there.

The lite version is all wizard based so I doubt there's an option specifically to finalise a disk. What you might have to do is append a small file and tick the finalise disk option when you burn that.

If it's Windows 7 then I don't think you need an application. Just right-click / finalise from Explorer.

Alternatively, I'd probably just re-burn the data to new disks making sure the finalise disk option was ticked at the start.

However, I'm surprised your Mac couldn't read it regardless of whether it was finalised. The entire point of being able to part-burn a disk is that you can read it, and append data to it later as you wish. If it's an old machine then it may have difficulty reading some brands of disk regardless. Does it have a DVD burner? Typical older DVD burners read DVD-R but not DVD+R, while some DVD drives won't read either format unless the dye is particularly good quality.