A few notes:
Firstly, you cant plug an external antenna into a powerbook unless your using a USB or some kind of external wireless device.
Secondly, a cantenna is directional. Meaning, it would suit your end of the link, but not the other end. The base station end must remain omni directional, so all can benefit from the signal it's providing.
Some options:
1) Get a beefier omni directional antenna installed on the base station for the complex. A good one would be a 'waveguide' antenna, which looks like a long section of gutter pipe with slots cut in it. Or just a colinea 'whip' style antenna. You'll want something that beefs the signal up around 7dbi to 14dbi, or something like that.
2) Get yourself a wireless base station, something cheap like a linksys or d-link or something, but something with the ability to plug in an external antenna. (the factory antenna's can unscrew).. then get yourself a cheap directional antenna. I made one out of a pineapple juice tin, however having it mounted outside my place caused it to rust, so I made a new one out of some rectangle aluminium pipe (3mm, gutter style)... It cost me about $10 usd all up with the bulkhead connector, etc.
Setup the wireless access point in your room/place, with the small directional antenna facing roughly where the base station is located, all good, super signal. Then wirelessly connect your powerbook to your own local base station. It would be a bridge, or repeater.
There is tonnes of information on the internet on doing wireless 'cheap'. Look for the local free wireless community web pages that have tips, tricks, howto's etc.
One I frequent is
http://www.e3.com.au/ (which is a western australia based 'freenet' site).
There are links on there on making antenna's, etc.
A good online shop is
http://www.freenet-antennas.com which is again, Australian. There are instructions on there on how to make a small aluminium 'brick' antenna (like the one i made), very easy, you just need a hacksaw and drill. This site also has info on other commerical antenna's.
good luck