You should remove the part of Building_number, since that is an attribute rather than an entity. Easiest is to use Mendix's autocomplete suggestions when making an Xpath (add the “[“ bracket and then press ctrl + space).
What exactly do you want to show in the reference selector, though? You say you want to show all buildings in the database, but for that you don't need to fill out an Xpath. Just fill out the correct data source on the “general” tab (the “attribute (path)” part, and on the selectable objects tab, set it to “database”. That should get the job done.