If it's Nortel, then you'll need your company to provide you with the Mac VPN client, as I'm reasonably certain it's not free. If you're really in a pinch, sometimes colleges (not knowing that they're not supposed to do this) will provide a link for students and staff to download the VPN client, on a publicly accessible server. I've scored clients that way. Google, baby.
As an aside, this is the kind of thing that people don't often think about when they say "Fuck Microsoft, I'm getting a Mac!!!1one!1!". Ditto for running Linux. Or BSD. Or...
It's not a knock on OSX (or Linux, or...), it's a realization that most of the rest of the world focuses development and support resources on PC's. I've got a laptop full of software that I need to use everyday that only works in Windows. I run Ubuntu when I'm not working. My servers at home are FreeBSD. The web/DNS servers I'm responsible for at work run CentOS.
It's just the way it is. Fortunately, I'm not a mouth breathing moron, so I don't have much difficulty in maintaining a trouble-free Windows box.