Aloha,
I have set up 128bit WEP at work and at home to work with the 6k and it works (kinda). One of the access points doesn't work with the 6k. I always use HEX for WEP key input, you just have to make sure you enter it correctly, both on the AP as well as the 6k. Easiest is to write the WEP key on your desktop and then copy paste it to both your 6k and the admin page of the AP. That way you will be sure both have the same key. If you don't know whether you have an Open or Shared system, try both. If it is set up wrong, all the feed back you will get from the 6k is that it can't establish a connection and that the WEP key might be wrong. But the WEP doesn't have to be wrong. The application seems to only have one error message.
But it should work... (I used an Linksys WAP11 and a BENQ Wifi Router, btw, NEVER buy anything from BENQ, crap company)