To unlock Olivia Fuller, you'll need to complete the quest "Dance Hall Days Pt. 1." This will require you to clear the new block and build Olivia's house. Once built, you'll need to collect the following items:

-Saloon (obtainable after completing "Dance Hall Days Pt. 4")
-Adventure Snake Basket (obtainable after completing "Walk Like an Egyptian Pt. 2")
-Trieste the Triceratops (obtainable after completing "A Page Out of Prehistory Pt. 3")
-Acrapolis (obtainable after completing "Virgil's First Edition Pt. 3")

Each of these items will require you to use the blimp to travel back in time to collect. Finish the required quest to collect these items to unlock Olivia!