Oh. Maybe my theme was already set up with something that enabled it already. The good news is that it's pretty much a matter of copying and pasting the template tag into a theme file, and depending on what theme you're using, it should be pretty easy to figure out where to put it.
In a perfect world, you would create a child theme instead of editing your parent theme directly for this plugin...but if you aren't looking to learn that much about WP, and/or if the theme you're using isn't likely to ever be updated, it's probably not a huge deal...worst case scenario is a theme update overwrites your Co-Authors Plus template tag insertion and you have to paste it in again.
Look for a file in your theme folder called post.php. If that exists, then that's the one to edit. If it doesn't, maybe post what theme you're using and we can try to figure out where your post template is.
Also, if you find page.php and you do want to create a child theme, it's SO EASY. Make a new directory with the name [existing theme]-child (or whatever you want), create a style.css file in it that contains this code:
Theme Name: cflyonline's Theme [this is the name that will appear in your WP admin when you activate the theme]
Description: Child of [your original theme]
Template: [directory name that the original theme is in]
@import url("../[directory name that the original theme is in]/style.css");
Then copy page.php into your new directory and edit the copy instead of the original theme file. Activate your theme in WP admin (Appearance / Themes) and you've got a child theme to work in.