Hello All! I hope someone can help...

I recently purchased a Canon AE1...I know I know...I'm going back quite aways....however....with the kind of photography I do...until I can drop about $2000 for a digital set up...I'm have to use this. (Actually I'm loving it; beautiful pictures!)

Along with that, I have a flash unit; a Vivitar 272 Thyristor. I've never used a flash like this.

Tomorrow night, I will be shooting a show indoor a church; not 100% sure of the lighting but typical for an indoor concert probably.

My questions are this: First of all, there's a small slide thing just above where you were attach the flash to the hotshoe. It either shows M or 3 colors; yellow, red or blue. What does it mean? And what should it be on?

Second, there is a small dial on the side. Looks like I can set it for an ASA of 400 but no higher. Also there are f stops on the top of the dial...but I'm honestly not sure if I need to change this much. Can anyone offer a little insight into this as well?

Also, I can adjust the flash based on angle; 45, 60, etc. All flashes I've ever shot with have been straight ahead. When would I change the angle? What would the results be?

If anyone can give me a bit of insight into this flash I would GREATLY appreciate it. And the show is tomorrow at 6 so I'll be checking back to see if anyone can help.

Thanks so much!!!!