I did it all perfectly right, that's how I installed many other distros in the past onto my computer, without needing to change anything (e.g. didn't need to change boot priority in BIOS). Anyway, now, for some unknown reason, I had to change the BIOS. Under "Hard Drive" you can see the USB stick if plugged in and put it into the first place. From there I was able to boot the USB stick and all is perfect now.
Don't know why,but hope it'll help other's too.
Linux Mint 15 Olivia / 64-bit / Cinnamon