If no XPage is associated with the documents form, the action from the viewPanel will try to open the document in a XPage of the same name as the form. If no Xpage of the same name exist, the action will fail.
So you might want to create a XPage with the same name as the document'f Form - adding controls that can be bound to the data sources on the Form.
Or, you can open the Form properties and select the XPage from the 'Display XPage instead' drop down to get things going