Can't I resize my existing partition?
When resizing the partions you will need to also boot to an external drive or DVD. Once booted to that you should be able to resize the volume without any problems.

If you don't have an external drive with OSX loaded on it. You can download the Recovery Disk Assistant (http://support.apple.com/kb/DL1433) which has disk utility on it. This will just load onto a flash drive.