This may not answer your question, but the WL-138g is a horrible network card from my experience. It was a big pain to set up and use the way I wanted to. In the end, I'm now not using ASUS drivers at all but drivers directly from Marvell found through device manager's update feature...
I don't know about it not being detected--that's a tricky one. Didn't the card come with a CD? Try downloading the drivers and using them and see if anything's detected. If it's not, try a different card. There really is no hope for this one in terms of improved features or any of that. It has worked alright for me lately, but I am using a third-party wireless program (Odyssey Client) to keep stability. I'm not sure if the stability problem is that of the card or the router (probably the router) but still... I'd get a card more well-known, probably one with afterburner support too, but that's not completely necessary
My advice may be the best you'll get when it comes to this card. Sorry about that. Just warning you--don't keep it if you still can get a different one. I wouldn't wait around for a reply here.