You can use iPad-compatible TumbleBooks by accessing TumbleBooks through the DCL website on your iPad/iPhone and going to the Story Books section.
Just below the "Story Books" heading, there is a link called "Click here for iPad-compatible titles". Click on this link, and the iPad books will be sorted for you. Now, simply click on the "iPad" button to launch the book of your choice! The book will appear in a small window at first, but to expand, simply click on the "full screen" button.