I shot some video with my D300s the other day at Niagara Falls and when I imported into Aperture I am not able to play the avi file.

What could I do to get them to play on my mac with SL 10.6.8?

Have you tried using VLC?
That application hasn't failed me yet and I play a lot of different types of videos, many with an avi container.

VLC didn't work. Says it can't open because it does not recognize the file.

Ouch.........

Then I would try a bunch of different video converters to see if any of them can do anything with that file.
Did you check the file name.........is it with a .avi at the end?

For converters, if you still have Visual Hub - that's still my favourite, evom is pretty good and very fast,
you can also try D-Vision 3 to see if the file needs to be repaired
or try to import it into Simple Movie X

Just a lot of tries hoping the file is not corrupt.

Handbrake + VLC. If that won't convert it, I'm afraid you may have a corrupted file. Will it play natively within the camcorder?