How do I create a 3rd partition for linux?

[Apple2000]

I already have a mac partition and a W7 partition. I burned a ubuntu install disc. How do I create a third partition to install linux to without deleting any of my other partitions?

I am on a 2011 iMac.

 

[treestar]

You should still be able to split the Mac OS Extended partition in Disk Utility. It's a simple click and drag.