Your contacts need to be stored in your phonebook in the following format to be shown in the app:
+ 'Country Code' - 'Number'
For example: The country code for United States is +1 and a 10 digit US number is 8000100200. Thus a US number will be saved as: +1-8000100200
Note: You would also need to ensure that you're connected to the Internet for Ringo to import all your contacts.