Hi Dave,
The WAIT modifier is used with serial input commands not with the serial output commands. It's used to only start "capturing" data starting with a certain character(s).

Simply have your SERIN2 command capture the operator in a BYTE variable. Then, when you have Num2 as well you can use a bunch of IF-THENs to determine what the operator is, and act upon it accordingly OR even better, have a look at Select Case.

Code:
SERIN2 PORTC.5, 16780, [OP]    'MODE 16780 = 2400 BAUD INVERTED
Every character, A,B,C, +, -, * etc is represented as a single byte number. So all you need to do is something like:
Code:
IF OP = "+" Then
  Total =  Num1 + Num2
ENDIF
And so on.

Hope that helps.
/Henrik.