Resetting MBR on iMac
Boot from an install DVD or USB key (If you have one).

From the first installer page choose Disk Utility from the Tools menu.

Check to see if it finds your partition.

Verify/Repair it.

Then, from Tools, choose Startup Disk, and see if you can set your system partition.

If you don't have an install DVD or USB key there are plenty of instructions around on how to do it.

iPartition can change partition schemes without wiping the disk if it comes to that. It's a great tool to have, I've used it a lot for various disk activities over the years, with Linux, Windows, and OSX partitions.

You need a GUID disk to boot OSX normally, not sure how you had it working with an MBR. Disk Utility will normally tell you what you have and how to correct it, if it can.

This is similar: HD accidentally reformatted to MBR, how to get back to GUID?

Long story short - make sure you back up your disk, you're probably going to be reformatting it.