Mylon Well, that turned out to be a lot harder than I thought it was going to be!
I was going to say just use On any Click, then Left button Released... but it turned out to be a lot more complicated to make the type of interface you want.
So, take a look at my capx: http://www.rieperts.com/games/forum/MouseClickTest.capx
It can tell the difference between clicking, double clicking, and click-and-drag for both the left and right mouse buttons. The scroll wheel zooms in and out to where the mouse is pointed. Having a zoom feature made it even harder to have the mouse click and drag work properly to select units...
A double click always registers as a single click first, then a double click (if the second click happens within the threshold time, and within the threshold distance of the first click). That is pretty universal how mouse events fire. If you really wanted a double click to not count as a single click first then you would always have to wait until the threshold passes before you could process a single click. So, factor the order of events into your UI design. (a single click should select a unit, a double click would then select the unit and allow a secondary action to happen).