On you iMac.
Go to Amazon.com, from the menu on the left select Books then Kindle eBooks. Now you should have a bunch of Kindle related links near the top of the page just under the search bar. All the way to the right is Kindle Support. Select it.
On that page you will find a Kindle for Mac link. Go ahead and click on it and continue on that route until you have downloaded and installed the Kindle application on you iMac.
The first time you start the application it should ask you for your Amazon.com user and password. Or there will be an account setting in the menu, or something else. I forget. Point is you have to set it up so it can access your Amzon.com account.
Once you've doen that all the books you have bought from Amazon should show up in your archive page, just like they do on the iPad. Just select the ones you want to read and they will download to the iMac.
Hope that answers your question.