First go to your 'manage your kindle' page at Amazon. All the way at the very bottom of the page is a link 'manage synchronisation between devices'. Click on this and make sure it is set to 'on'.
Make sure your Kindle has wireless turned on and your iPad has wi-fi or 3G access. When you finish reading on one device (and preferably exit the book), when you open it on the other device, it should ask you if you want to go to the last page read. Choose yes and it should take you directly to the page you were on with the first device.
You can force a sync on the Kindle by going to Menu --> Sync and Check for Items and on the Kindle App on the iPad by pressing on the circular arrow on the bottom right of the screen when you're in the book.
You don't need to leave wireless on all the time if you don't want to, you can just connect when you want to sync. If you use that method it's best to make a habit of doing a manual sync, each time you finish reading.