Adafruit does have a great tutorial on how to set this up, but I wanted to make this a bit more specific for my students and their projects.
Step 1: Adding an Additional Board Manager URL
There are many different board manufactures out there and often you need to go through some effort to get all the settings and files needed to get them to work properly.
The first thing we need to do is tell the Arduino IDE where to look on the web for the additional board information. We do this in "Preferences" and then add the URL to the list under "Additional Board Manager URLs". Notice in the picture that you can have several different URLs in this field. You just have to separate them with a comma.
NOTE - If you're using an older version of the IDE you may need to edit the preferences.txt file directly
The line you want to put in there is:
Hit OK when done.
Step 2: Installing the Board
Go to Tools > Board > Board Manager (see the pic).
Scroll down the list and look for the Adafruit AVR Boards.
Click it and the Install button should appear.
NOTE - you might already have a board manufacture installed, but the board you want doesn't show in the list. This might be because you have an older version and need to update it.
After it installs, close the window.
Step 3: Adafruit Metro Board Installed
You should now be able to see the board in the list. Notice it doesn't say "Mini". That's ok, its the same and will work.
Step 4: Download USB to UART Bridge VCP Drivers
Adafruit's interface chip is a little different than your typical Arduino so you will need to get a new driver. This is the VCP driver or "Virtual COM Port".
What this means to us is that the IDE now knows about Adafruit Metro boards, but your computer doesn't know what to do with it. It won't see it or show up when you plug it in.
Go to Silicon Labs and download the driver.
Extract the zip file and you should see a couple executable files. Run the one for your system.
** If you're not sure, look up "how do I tell if my operating system is 32 or 64 bit?"
Step 5: The Other Virtual COM Port Driver
Step 6: Finished!
At this point you should be able to plug in your Metro Mini, go to Arduino IDE and see the new port.
Keep in mind that the Arduino IDE doesn't always attach a convenient name to the ports. You somewhat have to assume that any high numbered port is likely your board.
Try uploading a sketch. It should work fine now.
If you like invention and video tutorials I also do a lot of this on my youtube channel
.dissonance on YouTube