[Blossom]

To get sideloaded books to show up in the carousel you need to remove the pdoc tag from the mobi book.

You can do this in Calibre.

From Calibre's Book Conversion window click on MOBI output
Find the box that says Personal Doc Tag: [pdoc]
Delete [pdoc] from the box and reconvert book.
It should now show up with your other books on the Fire.

You can set Calibre to permanently do this in
Preferences/Output options/MOBI output.
Find the box that says Personal Doc Tag: [pdoc]
Delete [pdoc] from the box and click apply at the top.

Note: If you don't care about the books showing up in the carousel then just email them to your Kindle. This will use the Personal Documents Cloud and you will be able synch those without modifying them in Calibre.