The card was used & that's the way I received the antenna. So, I had always thought that it just pushed in. Anyhow, I got the plug out w/ a pair of pliers & stuck it back in the card. Now it looks more like the type of plug I recognize.
I hope it stays in. I had several WMP54Gs and eventually most of the antennas fell off. I tried to press fit the back in, but they fell off again in time. No longer using them switched to 802.11N. Good luck.