HD Audio cable won't fit into sound card

Hi, I've just about completed my first new build however have come across a slight problem.

The HD Audio cable from the case which would normally go into the motherboard, I have purchased an Xonar DX2 sound card however there doesn't seem to be a connection point where I can fit this cable into on the sound card. A cable came with the sound card which fits into the side of it, but where is the other side supposed to go, it's very short and there doesn't seem to be a place for it. I thought that the audio cables that came attached to the case were supposed to fit inside the sound card, do I need an adapter or something?

Any help would be appreciated.

Thanks.