GraphLab Canvas currently runs on a random available port (<a href="http://compnetworking.about.com/od/tcpip/p/port-numbers-0.htm">port 0</a>) and thus cannot be specified. It also will only listen on localhost and not other network interfaces. When the browser opens, you can see which port Canvas is listening on by looking in the URL bar of the browser.
Thank you for the feedback -- I can see that it might be useful to be able to control the port and interface that Canvas is running on. We will consider this for a future release.