In the Catalog Explorer -> browse to the procedure by clicking open catalog -> schema -> stored procedures. THen click open the node of the stored procedure and you should see a node #0. Right-click it and select Change from the context menu. You then can change it to 1 or 2 or another value. (but 1 or 2 are sufficient).
After you've done that, you can add it as a retrieval procedure.
Hint: the Catalog Explorer is usually located on the right side of the window