Address Lookup - Address Entity not appearing in 'Target Record Type' list - CRM Online

Hi,
I am trying to add an 'Address' lookup in the Appointment form.  The requirement is that, instead of entering the address manually in the Location field, user should be able to select an address from lookup based on the selected Account in the Regarding field.  (Same way as it is done out-of-box in Contract form)

Problem: When I try adding the lookup in Appointment form, the Address entity does not appear in 'Target Record Type' list.
I also checked other forms, address entity does not appear in the target record type there as well.

Please advise what could be the reason.  Also, how the above requirement can be accomplished. Any related articles or blogs will also be appreciated.

Thanks in advance,
Saima Munir