Also, if you go to the Sitecom website, they only provide an old driver (v1) probably working with older OSX 10.4 and 10.5, for sure it does NOT work with LION and Mountain...
For your information, this is the interesting info about the device:
802.11 n WLAN:
Product ID: 0x005f
Vendor ID: 0x0df6 (Sitecom Europe B.V.)
I've completely resolved as follows:
1) Download the file RTUSB.dmg for RT2870/v126.96.36.199 directly from Ralink here:
2) Install and reboot
3) As root:
edit Info.plist by adding the following section:
<key>Sitecom23 - WLA3000</key>
4) Reboot again, insert the WLA-3000 and enjoy!
Attached is the patched kext (based on v4.6.0). Tested OK on LION 10.7.5 / 64 bit.