The driver's manual says that there is a 12 volt plug on the passenger's side just above the little cargo net. Do you have it? Is it sort of hidden?
I might be wrong, but if you have an ashtray and lighter in the centre console, then you don't get the footwell 12v outlet. As Dan says, you can't miss it if it's fitted.
I have a 12 volt gig-like socket in the boot of the SL but nothing on the passenger's side of the SLK. There is a small plastic plate there that probably covers the hole.Corkscrew,
the 12-V-plug is exactly at that position in my car which you have described. But there might be a correlation concerning national options.