There is one other thing to keep in mind with the Hibernate plugin for 
Middlegen. In the build.xml file, on roughly line 145, in 
the "middlegen" target, you may need to modify these lines:
  schema="${database.schema}"
  catalog="${database.catalog}"

I had to delete the "catalog=..." line before I could get things 
working with a DB2400 database. It seems that even if this property is 
empty, passing it as a parameter may cause an error that middlegen 
cannot find any tables. (The forums mention you may need to remove one 
or both the lines to get things working). Beyond following Chris' 
suggestion, adding a few "<table..." tags and tweaking the default 
generated package names, it seems to be working for me.