I'm very confused where exactly do you meet up with your tour guide in central paris ALL IT SAYS IS CENTRAL PARIS.
Paris is a very large place and many areas could be considered as the center, so you will have to contact the tour agency and find an exact location.
Viator only works through local operators in Paris, that's why they don't mention the address where the tour starts. It's almost for sure that the local operator in Paris is Cityrama and if you look at Cityrama's website, you'll see the address.
You can call Viator, the phone number is at the link below but anyway, when they say central Paris, in the case of tour operators, in general, it means between the Louvre museum (rue de Rivoli) and place de l'Opéra (Garnier), which is very central.
