Nikon D90 multiple exposure

On my Nikon D90, it won't let me do the multiple exposure mode. It doesn't allow me to select it on the menu, and when i try to, it says "This option is not available with the current settings." Can anybody tell me why/what settings to use to make it work? I can't figure it out.

Is it set on manual?

Page 121 of your user manual tells you how to do that.

If you have any of your cameras setting in an auto mode, some features are not usable. You will notice as you read the instructions that you have to shoot in the RAW mode

None of that worked