I added the following expression:
([Member ZIP Code]&':'& Sum([Member Count]), Sum([Member Count]))
I select this as a Text as a Pop-Up and deselected for Longitiude and Latitude.. also unchecked "Pop up Labels on Presentation tab.
Now the only thing that shows up is the sum of Member Count. Can't get the "Member ZIP" to precede it like in the Dual expression above.