No guide or driver will meet you. Before you go, open www.roamer.travel/redeem on your phone and enter your Viator booking reference. The tour opens in your browser, no app needed. Then go to the flagpole in the center of the Zócalo, face the Palacio Nacional whenever you like, and press play.
The tour ends at the Palacio de Bellas Artes, the white marble opera house at the edge of the Alameda Central. From here you have the park, cafes, the Torre Latinoamericana across the street, and Bellas Artes metro station right beside you.