The Settings > Import screen lets you browse and import GPX and CSV formatted files into PocketEarth. To use this the files must be put in the app via iTunes by choosing your device, choosing the Apps tab, and finding PocketEarth in the File Sharing section.
An easier alternative is to email yourself the file, and open the attachment on your device. When you choose PocketEarth, it should show you the Import Options screen. After you tap the Import button on it, the imported data can be found and selected in the Favorites screen, usually under the My Routes tab. (You can get to the Favorites screen by tapping the black and white Star button (4th button in the main row of 5))