Upgraded Movable Type from version 3.11 to 3.15. Downloaded just the "upgrade distribution file" rather than the full distribution. Since my PowerBook is now my blog server, using the Mac's Finder, I just opened up my mt folder and the MT-3.15-upgrade-en_US folder, selected all files in the 3.15 folder except the extlib folder, and dragged and dropped the 3.15 files into the mt folder, replacing the old-version files. I then I opened both extlib folders and dragged and dropped all the 3.15 files into the installed extlib folder, replacing. My existing extlib folder contained additional "external libraries" that I had added earlier so this is why the extlib folder had to be updated that way (as per the upgrade instructions). Then I did a full rebuild of the site.