How can I connect my xbox 360 slim to a public Wi-Fi network?

I live in an apartment and I don't have a router right now cause I don't have the money for it yet. I just moved apartments but am still in the same complex. I used my downstairs neighbor's network (it was unsecured and he didn't know I used it) and now I can't play xbox live without being signed out constantly due to poor connection (I can still pick up the network I used to use but it's too poor to stay connected). Now on to my point, my apartment complex has its own wifi network system where there is a network for each building of 8 apartments. These networks require a username and password which I know what mine are but I don't know if it's possible to join the network on my xbox slim (the newest black one). I can join it but it won't let me put in the username and password so I can connect to the Internet. Does anyone know how I can d� this or is it not possible and I should just wait and get a router?