Welcome to the Apple Support Communities. If you want to get content from your iPad to you iMac, there are a few things to try.
First check to the iPad to see if it is syncing with iCloud. To do this, go to Calendar app and click the Calendars button in the upper left hand corner. If it says iCloud, then all you need is to enable iCloud on your iMac if you are running 10.7 Lion or later. That preferences is System Preferences > iCloud, if you are
If on the iPad it says “On my iPad”, the following steps will guide you to get it back your iMac.
First, in iTunes, go to the preferences by clicking in the drop down menu iTunes > Preferences… Go to the device tab and make sure to check the box that says Prevent iPods, iPhones, and iPads from Syncing automatically.
Then follow the steps in the following articles to make a backup and the sync the data.
For backup instructions.
This one will assist with syncing your iPad to the iMac.