I have got jboss-castorjdo working with Postgresql.
What I did is :
1) load the jdbc driver in jboss using jboss.jcml mbean org.jboss.JdbcProvider
//put your driver here, org.postgresql.Driver for example
2) configure the DataSource with :
//here you can use your drivers XADataSource class instead
Attributes depending on you driver.
3) Add the castor-jdo mbean :
4) Add you database.xml and mapping files in the right place (jboss_home/conf/default in the example)
That should work...
May I also advice you to spend 10$ for the commercial jboss doc, which will give you invaluable informations (at least, this was the case for me !)