The Add DB Variable dialog is accessed by clicking on the Add DB variable link in the Info panel of the Edit Script dialog when editing an EMS script. With the Add DB Variable dialog open you can add code for a DesignBuilder Erl extension keyword by selecting from a fixed list of DB variables.
Note: DB "Variables" are more than variables in the strict meaning of the word as understood by programmers. They are actually a broad set of keywords that can used as part of DesignBuilder Erl code for a range of purposes to create loops, access model attributes, add Erl code depending based on model settings, including parametric variations etc.
Select the one of the DB variables listed in the right hand data panel by either a) double-clicking on the required variable, or b) by single-clicking on the required variable and then clicking on the Select 
icon.
When you press the OK button the code is inserted into the script at the cursor position.