DI Magazine uses a library system to manage all the issues. When you download the first free issue (DI 01) you not only get the magazine but also the Library structure. The library updates each time you enter the application. DI 02 (and other issues) is available from this library screen.
After you download and install the free DI 01, tap the center of the screen on any page and then tap the Home icon in the top left of the bar that pops up at the top of the screen. This will take you to the DI Magazine library. The library should automatically update with a preview of the cover of DI 02. All you need to do then is tap the Buy button and the new version will be delivered and installed into the library. When new editions become available they will be listed in the library too.
If the library doesn't automatically update to show the next issue of DI Magazine then exit the application (press the button at the bottom of the iPad) and re-enter the DI Magazine application to force and update of the library.