If you want to put a helper on a Field that is part of a composite field (eg Last Name is part of the Full Name composite field), you’ll need to add the individual field(s) as hidden fields on the form.
I like adding a tab that is labeled hidden, hiding the entire tab, then dropping in the fields I want hidden.
Form more information about composite fields –> http://www.magnetismsolutions.com/blog/paulnieuwelaar/2013/10/22/working-with-dynamics-crm-2013-composite-fields-fly-out-menu