By setting up MAMP, you can store video files on a computer, then stream them to FulGaz on your Apple TV. This is a great way to get around the file storage limitations on the Apple TV.
A great way to test that you've got the details correct is to try the address you're using in FulGaz in a web browser on any computer on your network. The setup screen looks like this in FulGaz:
If we wanted to ride the Marin Headlands ride, this is the address that would play the correct video: http://192.168.1.5:8888/marin-headlands-extended.mp4
If that address doesn't work in a web browser on your network, it won't work in FulGaz either. Check you've got the settings correct and try again (your web address will differ from the link above).
MAMP server isn't turned on
You've got the default mac port set up (8888) but you're on a PC (it should be 80)
Videos have been downloaded with ID numbers instead of their names (in some browsers you need to right click to download and "Save download as")
Your router has been restarted, or your computer has moved location, both of these can change it's IP address.
If you have your Apple TV plugged into your local network using an Ethernet connection, it is possible that it will see a different IP address over Ethernet than it would over WiFi. This can be quite problematic to track down.
The best way to do it is to log into your router and see what IP address that sees for the computer running MAMP - that is the correct IP address. This can be tested using the troubleshooting tip above.
Further help and advice
The best place to ask for help is in the FulGaz riders Facebook group. You'll find people there with the same setup as you that can offer some tips and advice to get you up and running.