How do I manually add bookings (walk-ins, phone bookings etc.)
As a channel manager, myallocator is not designed to have bookings manually added to the bookings page. That type of functionality can be found within your typical PMS software. For Availability (inventory) purposes, if you receive a manual booking, you would simply reduce the number of rooms/beds for the room and night that you received the booking for on the Availability page.
If you use BookNow, you could actually add the manual booking through the BookNow widget while logged into myallocator as shown below: