Sure, if you want to join the game on the webclient use http://byond.com/play/ip:port

for the real client, use byond://ip:port into the web browser's address bar and it'll call to byond like it does for the "play" link clicked through an actual hub.

EDIT: Or you can go to your pager, and go to to the gear at the top right>>then click on Open Location

Or alternatively Control+O to open the box.

This box will take an "IP:Port" directly, or "byond://ip:port" as well, and some other combinations of things it will accept, but there's a lot of ways to connect to games, you should be able to figure at least one way out from this.