This is the code editor window. The fields of the current table are shown on the left, double-clicking them inserts the code to access the field assuming that the row variable is called 'row'. Underneath, you find a reference to the API that the custom object must specify. At the top, a list of templates allows you to quickly get the hang of it by looking at examples. Once your done hit the Test button and verify that it is working as expected. The Console^2 extension can be useful to debug your scripts. Finally click Ok. Custom objects are not yet psersisted.