Where do I download Xcode ?

If you want to quickly be able to look at the result of your InDesign publication in the iOS simulator, then you can download Xcode for free from the Mac App Store.

However, if you want to be able to create an Ad Hoc build and install it on an iPad, or publish to the App Store, you also need to be registered as an iOS developer. That Apple program is available for 99$/year.

Check Apple's developer web site for more details.

The iOS SDK required for Twixl Publisher Builder will automatically be installed as part of Xcode.