You can only connect to an access point if:
1. it isn't using WEP (ie, you see "WEP=No" in Kismet), and
2. it isn't MAC address controlled. Look at the IPaddress that kismet sees for the access point: if it shows up as "0.0.0.0", then most likely the AP is using MAC address control. If it has a genuine IPaddress, then there is a good chance that you'll be able to connect.
I have a Generic Wireless LAN setup that I created with the following parameters, which has allowed me to connect to every open AP that I've tried:
Non-specific ESS-ID: "ANY" (works for almost all open APs -- I've needed to put in the actual ESS-ID name for some D-Link AP models)
WEP: disabled
Obtain TCP/IP automatically (if the AP isn't acting as a DHCP server, you can sometimes get by with hardcoding in an IP address in the same subnet range as what Kismet gives as the AP IPaddr)
Obtain DNS automatically (again, if the AP isn't acting as a DHCP server, you may need to hardcode a valid DNS server address -- I keep a few on hand in my notes)
No proxy (usually, only commercial WiFi services require you to use their proxy server)
Once you have the Wireless LAN setup configured, then (make sure you've quit out of kismet -- you may also have to eject & re-insert the WiFi card) simply choose it from the little globe icon in the task bar. After a short pause, it should say "connected" and the globe icon will change from B/W to colored and the red "X" will disappear.
