It is not a problem if you have an international number. Instead of texting your Join Code to 735-343 you will text it to this number: (512) 308-3535.
Once you have enrolled and downloaded the Waldo Photos app, be sure to turn on Push Notifications. This will turn off text notifications to allow you to receive alerts when your photos have been found. You can do this in the app by tapping the three bars in the top left corner of the home screen, and then by tapping Notifications. Once you are on the Notifications page, tap on the gear icon in the top right corner, which will take you to Notification Settings. On the Notification Settings page, select "Push notification" under "Send Alerts Via..." and then be sure to tap Save in the top right corner to save your settings. You'll know that your settings are updated with the Save button disappears.