Which adapter/port do I need to Connect this Monitor?

[space-time]

I would try a DisplayPort to MiniDisplayPort cable. My Dell monitor came with one, and the 2012 mini pumps our 2560 x 1440 on this cable. But, 2012 mini is a newer machine, the thunderbolt port has the same shape as a MiniDisplayPort. I think such a cable should work for you. I am not 100% sure though. YMMV.

EDIT: perhaps this one, and then use the DisplayPort that came with the monitor?

http://www.monoprice.com/Product?c_id=104&cp_id=10428&cs_id=1042803&p_id=5714&seq=1&format=2

EDIT2: or a cable like this

http://www.monoprice.com/Product?c_id=102&cp_id=10246&cs_id=1024606&p_id=6007&seq=1&format=2