Step.Property("Script") = Step.Property("Script") & vbCrLf & "'more code"
would add code to the Script tab of a Run Script step.
The Script tab of the step properties dialog only exists for a Run Script step (its contents are stored in the step's Script property).
http://www.visualbuild.com/Manual/runscript.htm
The Step tab of the script editor dialog exists for any step and is used for editing a step's script event code (its contents are stored in the step's script property [note case difference]).
http://www.visualbuild.com/Manual/scriptevents.htm