Well without an essid you can't connect cause it won't be able to figure out which network to connect to and it won't get allowed either. Is this the problem or is your essid shown when you try with the wep key?
Unfortunately I can't help you much more as I haven't played around with WEP much
However did you compile the newest ndiswrapper from source or did you use a debian package? If you used a debian package and there's a newer ndiswrapper source package available it might be worth a try.
Also I think a search on this forum might turn up some results as well as I've seen a few of these kind of threads around.