Ok, if I understand what you want, here is what I would do.
Click “Macro” on the toolbar and select “Edit/Create”
Copy and paste the following into the window.
# Set default icon for any waypoints that don't match your if statements.
$_Symbol = "Default"
If $d_Terrain = 2 and $d_Difficulty = 3
$_Symbol = "Airport"
EndIf
If $d_Terrain = 3 and $d_Difficulty = 2
$_Symbol = "Bank"
EndIf
I picked two icon names that I know exist in MapSource. Change them to whatever you want.
Save the macro. Give it a descriptive name and remember where you save it.
On the Mapsource export screen, check the box that says “Use Macro for symbol generation”.
Click the browse button and select the macro you just saved.
Set everything else the way you normally do and save.
I don’t have MapSource, so I haven’t tested this with MapSource, but it works with everything else I’ve tried it with. If this doesn’t work, I can think of a couple of different things to try.