1. If you're running 10.4 or 10.5, boot in safe mode
2. Once booted in safe mode, find the program titled "Run Me First" and launch it. It won't visibly do anything, but is necessary to proceed with flashing.
3. Run ATI Multiflasher
4. Finally, open Graphiccelator. You will need to specify "ATI MultiFlasher" as the flashing tool, then specify the ROM file you are going to use. You should then be able to proceed with your flash from there.