Opening the firewall
You may have problems viewing your Share from an external computer if you have a software or hardware firewall active. The settings to Mac OS X's built-in firewall are below.
First, open System Preferences and click on the Sharing pane. Click on the Firewall tab. Open Firewall preferences.
Add a new firewall and match the settings below. Note that "50000" below is the setting entered in the Port field of the Preferences pane.
Don't forget to open ports in any connected hardware firewalls or routers.