Ah, I see. I would tackle this problem by configuring the lookup as an automatic lookup (if it isn't already) and then define the attributes from the "auto-extracted" list in your base search. That way, the base search of the data model calls out the values from the lookup table as required (or optional), so that they're available to pivot on.
lookups are diferent from attributes.
According the documentation attributes are the set of fields that the data model represents. They provide the fields that Pivot users work with to define and generate a pivot report. They can also be used to set up the definition of other data model attributes.
Object attributes are inherited from parent objects.
and this is my problem, I'm trying to create a pivot table where I have already a number of fields extracted (at the report level) but they don't seem to show on the data model.