Determine if an item is seleted in multiple value dropdown

I have a dropdown that has three options available. I would like to show the items in the legend only if the parameter for that item has been selected by the user.  How do I accomplish that?  I originally had something like this:

=iif( paramters!SelectTypes.Value(0) = "Type1",true,false), but this not doing what I thought it would do.

thanks

mhoran