Mac Boot menu?

Okay, well at my school I was given the "privilege" to be a Linux beta tester. I chose Macpup, because it is small, works on the old computers, and looks like a mac. I wanted to try it on my Mac, but I can't get to the boot menu. I've tried holding C, +OPT+SHIFT+DELETE, but whatever I do, it doesn't work. No, twice it got there, but it was just showing a tiny folder with the mac finder logo and a question mark flicking back and forth for a few minutes, then booted mac.
Help?

Specs below.