I made a partition on my iMac to install Ubuntu, which I did by DVD. I've been using it for a while but, I wanted to switch over to my Mac OS and When I restart my comp and hold down the "alt" button a purple screen comes up and gives me the option of picking Ubuntu or Mac OS using the arrow keys on my keyboard, but they won't move up or down for me and after 10 seconds it goes into Ubuntu.
Any advice would be greatly appreciated!