1 - Put a tabpane in the layout that you want to be the "MDI";
2 - Create a code module for each "form" with a show method;
3 - In the show method, each form will be loaded into the tabpane as follows: Main.TabPane1.LoadLayout ("Form layout", "Tab name")
4 - To close a "Form", you can use Main.TabPane1.Tabs.RemoveAt (Main.TabPane1.SelectedIndex)
Ready!