Workflows in CRM 2011 can only update fields which are visible on the form, if you create an entity update you are only allowed to pick the visible fields.
So what do you do if you want to update a hidden field?
- Go to the form, make the field/section visible.
- Save and publish the form
- create the update statement on the workflow, selecting the previously hidden fields
- Save the workflow and then publish
- hide the fields on the form, save and publish.