Located exactly in the middle of his Dock (13th from the left or 13th from the right), is the icon for Mixed In Key. Because this photo was taken before the sixth version of Mixed In Key was released, it's more likely to be Mixed In Key 5.
When asked how he can tell what songs will work well together in a mashup, Hardwell says:
"Most of the time when I'm creating a bootleg or a mashup, the idea just comes by itself. I'm using Mixed In Key to make a selection of all my tracks, and all the acapellas I have."
To make sure his tracks flow together well and are in the right key during his DJ set, Felix Da Housecat uses Mixed In Key 5. When asked if he's using version 5 already, he tweets "yeah buddy Traktor kidzz hooked me up, lovin it!"