To add help text to the automatically created ‘Help’ link which DNN adds to each of your custom modules you simply need to add a new ‘Resource Key’ to the control’s .RESX file. For example you may have a View.aspx control which is the default display control for your module. Simply click on the designer surface in visual studio for that control, go to Tools > Generate Local Resource. A new .RESX file will be created in your module’s ‘App_LocalResources’ folder. In this file add a new key ‘ModuleHelp.Text’ and set its text value to the text you want to show up in the help control when it opens. The value you give this key can be plain text or html.
DNN Adding Module Help