I just purchased a mac mini and it is not showing up in the associated devices list on my iTunes account. How do I associate it with my iTunes account??

Greetings,

Make sure you're signed-in with your Apple ID in iTunes on that computer: Add your computer, iPhone, iPad, or iPod touch to iTunes Match - Apple Support

If you are, sign out and then back in again.

Cheers,

Madison

Maybe this will help since I may not know what you mean

Manage your associated devices in iTunes - Apple Support

The above includes the following so did yo do that yet on the Mac?

When you turn on iTunes Match, Family Sharing, or Automatic Downloads, or when you download past purchases on your computer, iPhone, iPad, or iPod touch, that device becomes associated with your Apple ID. You can associate your Apple ID with up to 10 devices, with a maximum of 5 computers at any time. Each computer must also be authorized using the same Apple ID. You don't need to authorize your iPhone, iPad, or iPod touch.