The adapter has to be active display to HDMI(DVI... or whatever) most cost more than $15http://www.newegg.com/Product/Product.aspx?Item=N82E16814999034
(mini DP to DVI) < I would go with this if your hdmi monitor has DVI on it
BTW to do eyefinity with 3+ monitors, only 2 of the monitors can be non-DP. SO the 3rd and any after have to be
hooked up to DP.
The adapter you ordered is a passive, you need a active(which actually converts the signal).