Can anyone recommend a good way to convert an EPUB file to a MOBI on a Mac?

I just published my first epub to Amazon. It had a few minor errors, so I wanted to convert it to a MOBI myself and fix them, but I finally gave up.

I tried Kindlegen, but it doesn't work for me. I then discovered that you can do it with Kindle Previewer, but I got an error message saying I had to download some sort of Java program that I couldn't find. (Java and Apple both have information on it that just sends you in circles.)

I've read that Caliber can convert EPUB's to MOBI's. However, I also read that Amazon wants people to use its software. But would they even notice the difference?

Part of the problem might be Sierra. I've been using MAMP for web design (running El Capitan), and everything worked fine until I got some weird bug I couldn't fix. I upgraded to Sierra and MAMP 4.1.1, but I still couldn't get things working. So I tried installing the software myself, but none of the tutorials match my files.

I also had a security warning that initially prevented me from opening Kindlegen. So it kind of looks like Apple has more or less locked everything non-Apple out.

Thanks for any tips.
You don't want to convert to Mobi. You want to fix whatever is wrong with the ePub and then you convert with Kindlegen.
Amazon Kindles Nor Fire Don't even read epub....
Shameless plug: Install Sigil and my KindleGen plugin. (You'll also have to install ActiveState TCL, if you haven't already done so and you'll also need to reinstall Kindle Previewer, if you've uninstalled it.)

If you've installed Kindle Previewer in the default location (i.e. in the Applications folder) my plugin should work out of the box with the macOS KindleGen binary bundled with Kindle Previewer, unless additional security checks were added in El Capitan. (The plugin was only tested with Yosemite.)

Have you checked your book with the IDPF online validator?
Actually, the Fires can read ePub if you install an app to do so.

I don't know if you are replaying to my post or not, but since ePub is the source format, then it just makes sense to correct the errors in the ePub and then convert using Kindlegen.
I have heard of cases where Amazon sometimes rejects books converted using calibre. I would stick to kindlegen or one of the previewers, 2.x or 3.x. Previewer 3 has its own Java runtime and so doesn't need an external one installed.

I recommend figuring out why your Mac isn't running Amazon's software. The problems are most likely with your system. (I can't help since I don't use a Mac.)

Added: kindlegen produces Master MOBI files that include both the original MOBI format for older devices and the newer KF8/AZW3 format for newer ones. You really want that so that your readers experience all of the formatting in your book.
Wow, great tips; I'll give them a try.

I just discovered that iBooks isn't working properly, either. If I double-click an epub, it still opens up in iBooks, which works fine. But the iBooks icon on the taskbar has been replaced by a question mark, and nothing happens if I click it.

Anyway, it looks like Apple was taking a few hints from Mickeysoft when they designed Sierra. I'll have to work out a few bugs.
I had to download another software program to fix a KindleGen plugin bug, but it works great now. When I operate it, it points out two errors:

Warning: Missing TOC epub3 landmark: Use Add Semantics > Table of Contents to mark the TOC.
Warning: Missing SRL epub3 landmark: Use Add Semantics > Bodymatter to mark the SRL.

I don't understand what those are or how you fix them. I have files named nav.xhtml, toc.xhtml and toc.ncx

It sounds like "Add Semantics" wants me to add more information to the content.opf file.
KindleGen requires this information for generating the TOC for older Kindle models. Simply assign the Table of Contents semantics to toc.xhtml in Sigil via Add Semantics....

The SRL (=Start Reading Location) is used to mark the beginning of the book, for example the first chapter or the prologue. Even though Amazon KDP appears to have deprecated this landmark/guide item, it couldn't hurt to include it.

BTW, Add Semantics is referring to a Sigil context menu option that is shown when you right-click a file in the Book Browser.
Why are you making ePub 3? For ePub > Kindle, you don't need ePub 3. ePub 2 will do fine and it's easier to make.