Please note that only devices with Native AFP support will be supported on Boxee.
AFP will start to scan for network sources only when you first enter it (browse/ file sources), if you added AFP source then it will automatically scan and start resolving for sources as soon as you start using the Boxee.
For more information about adding File Sources please check the File sources article.
Don't forget to configure share permission locally on the device you are sharing from.
You can add it as a File Source from the Browser menu by click on the Plus icon
If you can not locate the host name in the list use the add by ip address option in the File Source menu