Adding custom field in SugarCRM 4.2.x is tricky. Most of the time it happens that after creating custom fields it is a panic to show it. There is a trick involved to do it smoothly. Goto Admin>Edit custom fields>select the file in which you want to create custom field. Once you are on the selected page create the custom field that you want using the tool at the left side of the page. Fill in the fields that are mentioned there. Select the data type that will define the custom field you are creating. It is to fill in the data in the database.

Now again goto Admin>Field Layout from here select the file in which you want to insert the custom field. Now here at the left side you can see the custom field that you have created and one blank row in the layout at the center. Simply drag and drop the custom field from the left side to the blank row in the center. Now click on the save button. Your custom field is visible in the module you added it. It also creates the custom table in the database simultaneously.