Welcome to the Board!
The + is a holdover form Lotus 1-2-3. Excel recognizes it, but doesn't do anything with it. It stems from Lotus' ability to enter a formula directly off of the 10-Key pad as opposed to having to goto the = key, then back to the 10-key. (It was a way to pull Lotus customers to Microsoft software...)
You can get a similar action with Tools-->Options-->Transition.
There was a fairly detailed discussion on this a few weeks ago, if I can find it I'll post the link.
Hope that helps,
Smitty