In the code you posted, there are two fields: S_F and Fac. Is location another field? I don't have enough information from what you have posted to give you a specific answer to how you should write the expression.
The thing to keep in mind is that IIF expression has 3 parts.
The first part must evaluate to a logical True or False.
The second part says what IIF should return if the expression in the first part is true.
The third part says what IIF should return if the expression in the first part is false.
For example this is a valid IIF expression (assuming you have a field called Location.:
IIF( Fields!S_F.Value = 0 And Fields!Location.Value = "Default", 0, 1)
This would return 0 if S_F is zero adn Location is "Default". Otherwise, it would return 1.