To use a wired siren with a Simon XT or XTi system:

Negative wire from the siren to the HW1- on the panel.
Positive wire to the + HW 1&2 DC out.

You will also need to program the panel to tell it you have a siren tied in to the panel:
    Go to program, System, HW Func.  Set it to Ext Siren profile.