Here's what I tried and it seems to work.
IF CONTAINS(LOWER([___Nav1_Value]), 'wt.mc_id=email') THEN 'Email'
ELSEIF CONTAINS(LOWER([___Nav1_Value]), 'wt.mc_id=display') THEN 'Display'
ELSEIF CONTAINS(LOWER([___Nav1_Value]), 'wt.mc_id=paid search') THEN 'Paid'
Else 'No Search'
I think you are on the right track with CONTAINS. I also added the LOWER function to change the string to lower case and then compared it with values that were all lower case (e.g. 'wt.mc_id=email') as CONTAINS is case sensitive.
Hope that helps!
By the way, Amy, welcome to the forums!
Joshua, you did it! Thank you so much for helping me cross the finish line on this one. Your feedback was exactly what I needed. And thank you for making my first experience in the forums such a positive one.