Hi there Vaccaro81,
Thanks for joining us in Apple Support Communities! We understand messages aren't syncing, and you're asking how to view the messages from your other devices on your new iMac. We're glad you're here, and we can provide some guidance.
To start, you'll want to be sure you signed into Messages with the same Apple ID you use on your other devices. Learn more about setting up Messages on your Mac here:
Set up Messages on Mac
Next, in order to keep your messages updated on all your devices, you'll want to check to be sure you're using Messages in iCloud on your Mac. Have a look at the 'Turn on Messages in iCloud' section in this link for more information:
Use Messages in iCloud on Mac
Before turning on Messages in iCloud on your Mac, you may want to double-check your other devices to be sure they're using Messages in iCloud, too. Find out how here:
Use Messages in iCloud
Also, if you want to receive SMS/MMS text messages from your iPhone on your Mac, you can set up text message forwarding with the steps in this support article:
How to forward SMS/MMS text messages from your iPhone to your iPad, iPod touch or Mac
We hope this information is helpful. Don’t hesitate to let us know if you have any more questions.
Take care.