The Add EMS Variable dialog is accessed by clicking on the Add EMS variable link in the Info panel of the Edit Script dialog when editing an EMS script. With the Add EMS Variable dialog open you can select from a fixed list of standard EMS variables.
Select the one of the EMS variables listed in the right hand data panel by either a) double-clicking on the required EMS variable, or b) by single-clicking on the required EMS variable and then clicking on the Select 
icon.
When you press the OK button Erl code is inserted into the script at the cursor position.