Sony Ericsson W810

I bought a Sony memory stick pro duo 2GB memory card model MSX-M2GN off of e-bay, but when I plug it into my phone, it does not recognize it. I played around with it, and got the phone to recognize, but it won't recognize any music that I have put on the memory card from my computer. When it finally did recognize the music, it would not play the music, and the files would no longer show up on the memory card after trying to play them. What is the problem??

Thanks