Here is what I did:

Example:

SELECT Manufacturer, Model

FROM CARS

WHERE Manufacturer = @Manf AND Model = '2 door'

Here is what I wanted a prompt for Model = '2 door' or Model <> '2 door'

I created a prompted with two option

Label Value

2 Door AND Model = '2 door'

Not 2 Door AND Model <> '2 door'

So I took the query and placed it in an Expression - from the Dataset.

="SELECT Manufacturer, Model FROM CARS WHERE Manufacturer = @Manf " & Parameters!Model.Value & " "

You now have a prompt to handle EQUAL to and NOT EQUAL to