JT2Go for iOS uses the standard iOS "Open in" functionality of iOS apps to load .jt files.
This "Open in" functionality is found in many apps that are able to download and share files. Built in apps that can do this are the Mail app and Safari app. Simply download a .jt mail attachment in the Mail app, or download a .jt file in Safari, tap on the file, and when asked which app you'd like to open it in, select JT2Go.
Other 3rd party apps that may be useful in getting jt files onto your device are such apps as Dropbox and FileExplorer.
- Thanks to Steve Menyhart for providing this Answer -