Did you rename the RUU to PD98IMG.zip? Rename the RUU, put it in the root of your SDCard (not in any folder). Boot into HBoot. When it asks you if you want to update, say yes.
I am more knowledgeable on flashing custom ROM's than RUU's so it is safer to wait for confirmation from one of the more senior members here.
But, we are almost there