Photo Gallery Website: Error opening URL

Hi! I’ve been trying to figure this out for a few days now and I have a notably tough time with actionscript so I have no idea how to determine what my issue is here. I am using the tutorial for the thumbnail photo gallery with XML and I’m trying to Set up three sections of my site with three different galleries. Problem is, I can’t even get the first gallery to work - I keep getting an Error opening URL “undefined”. All of my photos are in the same folder with my fla and swf. Might anyone know how I could fix this issue? I’ve been looking all over the internet for an explanation I can understand but I’m coming up short. Thank you!!