if u'll use the "Search Query Tools v2", u'll find out an interesting thing, the results of an MMD field contains a strain of guids, and a single strain for word,
the guids are the the guid of ur selected MMD and his childs.
the string is the label.
so what to so if you have a tree with lv1 say "Batman" and lv2 "Batman and Robin", and you want to retrieve only "Batman"?
if you put the "Batman" guid u'll get many resutls, if you put "Batman" u'll get many results.
looking in that awful string i noticed something, the really selected MMD has its guid twice, once usual, and the other with "0" (zero) before.
so just put in your query "0" (zero) before the guid.