Howdy Greg -
Since you are passing in a SequenceContext object into TestStand, you will need to convert to a PropertyObject in order to pass the sequence context into the "context" property of the ExpressionEdit ActiveX Control. This can be accomplished with an ActiveX method for the SequenceContext object. After hooking up a proper context, the expression browser dialog button should become enabled.
I've included a very simple VI which shows some of the key methods and properties you may want to consider for the ExpressionEdit ActiveX Control. I've also included a sequence to run this VI. Simply select "Run MainSequence" for execution in the sequence editor. I would also recommend setting a breakpoint at the <end group> so that you can take a look at the populated local variable.
Let me know if you have anymore questions.
Have a great day!
Regards,
Andrew W.
National Instruments