Just out of curiosity, where are you looking for the SLI setting? In the latest drivers, it's not even called SLI anymore.
Doing this from memory, so please bare with me. When you open the nVidia control panel, there is a list of categories along the left hand side. Under the first main group, the third sub-category should be multiprocessor or multi display setup (or something like that). When you click on this, you should have two options:
Multi Display
Multi GPU (Recommended)
Choose the Multi GPU and click Apply. Once you reboot, you should be in SLI mode.
Hopefully I have the explanation close enough, but if anyone can fill in the exact descriptions, please do. Otherwise, I will try to remember to edit this post when I get back home this evening.
-red