Aderson,
I'm not sure you need to define interfaces for your entities. The reason I used them in the Data Access tutorial is so I could use the same entities regardless of the data access method. For you, it may just over complicate since EF is generating your entity implementation code each time you change something in the designer.
I would model the relationship using the designer interface (.edmx) and see how EF builds the code in the designer file.
That being said I took a look at one of the designers on a project I did in EF. It was a 1-M relationship and on the one side, the entity contained a property:
public EntityCollection<ManyEntity> ManyEntity { get {...} set {...} }
So it seems like your interface is correct.
If you need more help, hit me up on Skype after work hours.
Good luck!