Thanks for the help.
Using your query, I set up a new browser folder icon for dimensions. In the where clause I inserted
object_type = 'DIMENSION'
union
select owner, 'TABLE', object_name, status
from all_objects
where owner = USER
and object_type = 'DIMENSION'
The new folder now contains the dimensions. Obviuosly PL/SQL Developer doesn't recognize a dimension object type.