Yay, I got it!
It was a combination of uncommenting that line, putting the .htaccess file in the right place, and setting AllowOverride All in the RIGHT section (I didn’t know there were multiple AllowOverrides. Hey, I told you I was an Apache newbie!).
On the bright side, I learned a helluva lot about mod_rewrite. Now I just have to figure out how to get my images in /img to load.