I've had some trouble with AutoMOD, but I found a way to fix it. I kept getting a mkdir() error every time I would try and install a mod. I checked and saw that the CHMOD of the /store/ directory was set to 0777. So, I tired setting the CHMOD of the /store/mods/ directory to 0777, and the installation of new mods worked.
I took another look at the AutoMOD installation guide posted here, and decided that the wording was a bit iffy. "AutoMOD must be able to create directories and write files within the /store/ directory. This is also a requirement of phpBB itself, so there should not be any difficulties here" led me to believe that I wouldn't have to worry about setting a CHMOD for the /store/ directory, since it's already set from when I installed phpBB. It should be noted that /store/mods/ also needs to be writable.
Also, the installation guide has a minor inconsistency: an "upload" directory no longer comes with the packaged installation files.