"I believe it has an older airport card."

That could potentiall be part of the problem. IF the Wireless router youre trying to connect to dosent support 802.11b and only 801.11g then that could be part of the issue... But most wireless routers are compatible with both if not only 802.11b if they are older.

As for interface robustness if you clcik on the airport icon in the menu bar at the top of the screen you should see it listed jsut below the list of wireless networks, jsut select it and it should trun on. Ofcourse the older cards may not have interface robustness.. it might only be the airport extreme cards.

I fyou have an older airport card youre prety much stuck with it as you powerbook probably dosent suport the airport extreem cards. You could however get a third party (Linksys, Netgear, etc..) external adapter that hooks into a usb port and use that.

Finally i forgot to ask... do you have security enabled on your router? If so do you have the proper WEP/WAP key set up to access it? IT could be that when you go outside youre actually using a nearby wireless network whose signal is faint as opposed to the one youre meaning to access, and then inside ofcourse you cant use yours.