You don't see all the functions listed on the right side when you are in the specify calculation window? After you put the If script step in your script, and click 'Specify', the dialog box should pop up.
You could also look here:
In FileMaker Pro, choose Help menu > Product Documentation > Tutorial
Ah! so stupid of me! I have to click 'specify' after the 'if' function is added!
I got it working now, thanks!
The different parameter options for the Get function do not appear in the list of function in the Specify Calculation dialog unless you first select Get Functions from the View drop down.