dbms_alert package

What is the proper way to use the dbms_alert package supplied in V7. I am trying to use the example in the Application Developer's Guide on page A-19
and it says to create a trigger on the emp table that looks like this:

create trigger emptrig after insert or update or delete on emp begin
  dbms_alert.signal('emp_table_alert','message_text'); end;

But when I do this from within SQL*Plus I get the following errors:

ORA-06550: line 3, column 1:
PLS-00201: identifier 'DBMS_ALERT' must be declared
ORA-06550: line 3, column 1:

What do I need to do to declare the dbms_alert package.

Mike Campbell                         Phone     : (205)730-3623
Senior Customer Application Engineer  Internet  : mlcampb1_at_ingr.com
Intergraph Corp.                      Fax       : (205)730-3300
Huntsville, AL 35894-0001             Mailstop  : GD3002