To add a stored procedure call, in Catalog Explorer right-click the catalog, schema or 'Stored Procedures' node and choose Create Call Definitions to Stored Procedures. This will bring up the Reverse Engineering Element Editor with the stored procedures to which you can specify a call definition.
You can also select one or more individual table valued functions, right-click and select Create Stored Procedure Call Definitions which will bring up the Reverse Engineering Element Editor.
The designer also offers a General Stored Procedure Call Editor. This is a special kind of Stored Procedure Call editor which allows you to edit the Stored Procedure Call which is currently selected in the project explorer. This editor is opened by selecting Project -> General Stored Procedure Call Editor in the menu, or by clicking the button in the toolbar.
The advantage of the General Stored Procedure Call Editor is that it keeps the current sub tab selected when you select another stored procedure call in the project explorer, which allows you to e.g. edit the mappings of several stored procedure calls without a lot of clicking through tabs.