It looks like the Hotel has built their own custom website and bookings system, separate to the main Hilton one (possibly to get round Hilton taking their $% of each booking).
If you want to use your HHonours number I would just book on hilton.com as you normally would, and ignore the hotels own website.
Direct hilton.com link:
http://www3.hilton.com/en/hotels/ont...FHF/index.html