I can't set the 'advance with dialog' in the step definition

[Kim, Go Joong]

Dear workflowers,
 
I created some tasks and complosed them in a workflow definition.
In the workflow definition, I can set the 'advance with dialog' option in
the user decision step,
but I can't set that in the dialog step based on my tasks.
The option field is grayed and not editable.
Why I can't set this?
 
Any suggestion will be appreciated.
 
Regards,
Gojoong Kim (Steve)