You won't be able to raise the multiplier on a 6600 anyway. it's locked.
If you want to lower it though it should be in your bios somewhere. Check your manual, alot of times with Asus boards if you have things set to "auto" instead of "manual" you wont be able to access some of the advanced options.
I have the P5N-E Sli and i cant find the 333mhz multiplier thingy option anywhere in the bios
Here's a link to the download page for the Asus P5N-E SLI. You can download a PDF copy of your manual from there. (It may take a while, but you can get a copy ). Once you've got the PDF it's simple matter to search on multiplier and find out that it's discussed on page 2-26 of the manual. Actually, start on page 2-21 because you want to go the Advanced menu and then select the CPU Configuration sub-menu.
That's for the CPU multiplier, but it's probably not what you want. My guess is you actually are looking for the spot in the BIOS where you can change the speed of the Front Side Bus (FSB) from the default setting of 266MHz (... to 333MHz??). The path to that setting looks to be Advanced -> Jumperfree Configuration -> FSB & Memory Config.