By default the Mac version of PTGui writes temp files to the /tmp
folder. This folder is not accessible from Finder, you would need to
open a Terminal window to access it.
But the /tmp folder is emptied upon boot, so a reboot of your mac should
make the space available again.
If you want to be able to access the temp files, create a dedicated
folder for at and point PTGui to it in Preferences -> Folders&Files.
Joost