Look here for a presentation by Steve Krandel on the topic of row-level security. You will need an additional security table mapping your BO user IDs to the grnated dimension values (BOUSER1 = UK, BOUSER2 = Germany, etc.), see also here and here
Keep in mind, you might be better off using [i]Virtual Private Databases /i such as in Oracle, which is totally transparent to BusinessObjects.
Also, realize that object-level security deals with columns, whereas row-level security… well deals with rows.
Andreas 
(BOB member since 2002-06-20)