Since ID 1 is a listbox (which can have multiple lines - and does in this case), you need to find the selected line. Your listbox even uses the extended selection style, so you can have more than one line selected. You will have to loop through $did($dname,1,%i).seltext to figure out which command you need to issue. The important question is: when should you trigger this? Judging by what I see in your example, I wouldn't think you needed to use extsel at all.
on *:dialog:mdx_dialog:sclick:1: $+(dialog_,$did($dname,1,1).seltext)
