Geoff--

I love Macs, but they can be persnickety when it comes to videos sometimes. I'm guessing you use Safari, so if clearing the cache doesn't work (you'd be surprised how often it does) check to see if you're using any extensions that might be interfering. A quick way to do that is to open Guitar Tricks in a private browsing window. Private browsing should disable all the extensions and such by default. If the videos play in a private window, well, there you go.

If that fails, I've found Firefox to be very friendly in handling things that other browsers don't care for.

Good luck--

--Matt