Your sub has 4 ohm impedance and I am assuming it is a Single voice coil judging by the connectors
The amp you have is a 2 channel amp so what you want to do is bridge the channels into one. In other words connect one wire to the negative of one channel and positve of the other. This should be obvious which one when you look at the engraved labelling on the amp.
The way you can only have it configured is 1 x 300W RMS because of the impedance of your sub (@ 4ohm)
Basically you aren't even swaeting your amp! You would need another one of those boxes and wire it in parallel to get a nominal impedance of 2ohms to get more power out of the amp (600W RMS)
If you want to go crazy then to get the most out of the amp with the subs you have you would need 4 of them to gain a nominal impedance of 1ohm!
With that kind of setup you will be hated by your neighbourhood within a 3 mile raduis!