In the Ribbon Bar when the dimension tool is active (either smart dimension or the other--sorry can't think of the name offhand), change the pop-down menu from "Horizontal/Vertical" to "Between 2 Points"
When you select the arcs of the slot, make sure you do not pick a control point, but try to select near the side where you want the dimension placed.
--Scott
For some pleasure reading, the Round Table recommends FAQ731-376