This is how I setup a trigger and equence for my tables:

create sequence s_npdes_WB_PARMS   increment by 1 start with 1 nocache;

create or replace trigger WB_PARMS_BEFORE_INSERT
 BEFORE INSERT ON NPDES.WATERBODY_PARMS FOR EACH ROW
DECLARE
 v_Id    Number;
BEGIN
 :new.Updated_By  := USER;
 :new.Last_Update := SYSDATE;
 IF :new.KeyID Is Null or :new.KeyID < 1 THEN
    select s_npdes_WB_PARMS.nextval into v_Id from dual;
    :new.KeyID := v_Id;
 END IF;
END;
/

create public synonym WATERBODY_PARMS  for NPDES.WATERBODY_PARMS;
create public synonym S_NPDES_WB_PARMS for NPDES.S_NPDES_WB_PARMS;

You do need public synonyms for sequences as well.