I managed to get mine through advice on this wonderful forum about 2 years ago, when someone published details of how to make an application and obtain the “code number” required.
The programme has certainly enabled me to catch internal connections from International flights in under 1 hour..
Coincidentally, I have the leaflets about current application process on my desk. If you can Google and find:
CBP Publication 0101-0513
CBP Publication 0000-0806
There is also the website – wwwglobalentry.gov