I don't think I was clear. I'm not looking to display and actual '+' sign, but more like an "if 'a' and 'b' then 'c'" The user will select a value (e.g. store location) from the drop down list in say cell a1 and select another value (e.g. vendor) from another drop down list in cell b2. The form will generate a value (e.g. shipping location) in a 3rd cell.
If what you posted will do that, then I am confused about the formula you posted.