Q: how do I get old imessages from my ipad onto my imac

Greetings spmckm,

Thanks for using the Apple Support Communities!

From your question, it sounds like you have set up Continuity between your iOS device and your Mac for Messages as described in the following article.

SMS

With Continuity, all the SMS and MMS text messages that you send and receive on your iPhone can also appear on your Mac, iPad, and iPod touch. You can also reply to your contacts from whichever device is closest to you, including your iPad or Mac.

You can start a conversation in the Messages app or click a phone number in Safari, Contacts, or Calendar.

Set up SMS and MMS with Continuity

1. You need iOS 8 or later on your iOS devices and OS X Yosemite or later on your Mac.
2. Sign in to iMessage with the same Apple ID on your iPhone, your other iOS devices, and your Mac.
3. On your iPhone, go to Settings > Messages > Send & Receive. Add a check to both your phone number and email address. Then go to Settings > Messages > Text Message Forwarding and enable the device or devices that you want to forward messages to.
4. Look for a code on the Mac, iPad, or iPod touch that you enabled. Then enter this verification code on your iPhone.

Use Continuity to connect your iPhone, iPad, iPod touch, and Mac - Apple Support

It sounds like you have set up Messages correctly, but older messages aren’t loading. If this is the case, this is normal behavior. Messages will appear across all of your devices as they come in, but messages sent or received prior to signing in to Messages on your Mac will not be displayed. 

Have a great day!