When Lead is configured as a configuration option and you want to view Leads in the Reservation Form on the Resource calendar, you must add an extra Dimension Junction list to the Reservation Type view.

Be aware that there are now two B25__ReservationContact__c dimension junctions to select, because this object relates to Lead and Contact.

Once the new dimension junction display setting is added you can now see Leads and Contacts on the reservation popup on the resource calendar