When the field is disabled it means that the object is not there. Make sure that you create the object attach it through the reference before you show the page.
As visible in the figure, in order to make the data fields editable, I have used the many to one function between BP_Attributes and ProjectAttributes (where project attributes is a superficial entity having other entities as its elements).
As you also can see, I have changed the direction of association for the BP_Attributes. But when configuring the attributes on the page, I dont see the association for (BP_Attributes_ProjectAttributes). However, the other entities are visible in the attribute (path) of the data source.
This situation persists even after changing the Association type to reference.
An association is a possible relation within Mendix Thus by creating a ProjectAttribute object doesn't result in an BP_Attribute object.
you need to create that object as well and set the association be tween the 2 objects