This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
I don't see a FK mapping of conf_Id in your User class, so how would Hibernate know to put a value into that field?
Also, be careful about id naming and capitalization. I got confused how in User you have "id" to a column called "ID" and in the conference it was "conf_Id", the normal default way, if you can change the database is to call the attribute in the class all lower case "id" and allow Hibernate to automatically combine the class name and id name like Class=Mark and id is "id" Hibernate will give you "mark_id" as the PK field name.