Any number between 49152 - 65535 should be fine. eg: 64049
Yes if you have your OSX firewall active then you'd need to add it as suggested here OSX Firewall settings for Tiger

If your wifi doesn't have UPnP abilities, then you will need to port forward. All wireless devices use NAT firewalls. You can see if you can find your model here http://portforward.com/routers.htm (instructions for windows users but mac shouldn't be too different), but 1st you need to set up a static ip if you port forward; use http://www.blizzard.com/support/wow/?id=aww01180p or else http://www.sonic.net/support/ss/mac/osx_staticip/
If you have the manual to your device, check that 1st.