I think it's nice that the labels.txt file is created by sigpro. Very cool. But where does it put the file that it creates? This might be an important bit of information to include in the documentation.

If the labels.txt file is supposed to be in the gallery directory, it's not. Am I looking in the right place? Could this be a permissions issue?

Please read carefully the documentation -at the link below- and check the Individual captions/labels on popup images paragraph.
www.joomlaworks.net/content/view/51/41/

So, I've checked the paragraph you mention - no reference to where the txt file is located.

Please could you clarify?

Thank you

The file is created inside each gallery folder.

I don't see the file there, I actually placed one in there and it still doesn't work.