There was a change to the USB system in OS X 10.5 (Leopard) that makes it impossible to upload new firmware to the Make Controller.  Fortunately, there's a pretty simple workaround to this.

UploadHelper is a simple AppleScript-based application that makes a change to your system to allow the upload to work.  Then when you've made the upload, run it again to return your system to normal.  That's it!

Grab it at http://www.makingthings.com/resources/downloads/UploadHelper.dmg

Special thanks to Brent Burton and Seth Robinson in the MakingThings forums for putting the original source code for this together.