I'm having similar problems with my plug and a few different USB cards I have tried.
The one that I'm trying at the moment is the TP-LINK TL-WN321G. It shows up in lsusb as:
Bus 001 Device 019: ID 148f:2573 Ralink Technology, Corp. RT2501USB Wireless Adapter
I have followed the instructions on http://wiki.debian.org/WiFi/rt73#rt73usb
for installing the rt73usb driver but it seems no love.
I, like you, am unable to see the installed ko file in /lib/modules/18.104.22.168/kernel/drivers/net/wireless so I suspect that I may also have an installation problem.
Modprobe also throws back: FATAL: Module rt73usb not found.
Did you have any luck solving your problem? Thanks.