I\'m an NHibernate developer trying to give Entity Framework a shot in a hobby project. I\'m used to specifying mapping data in code using Fluent NHibernate. Pardoning Micro
Your junction table must be what MS calls a 'pure join table' - it must contain only the two foreign keys, and no other columns. In your case, that means you must delete the primary key column.
When you add the association to the model you choose each of your two tables and choose "many" on both sides of the relationship. When this generates the database script a join table with only the two keys will be created for you.
By the way: If you don't like using Model-first and would rather code research "Code-First" development for the Entity Framework. You can also do Database-First if you prefer that.