You don't. You will go kite-Surfing at Cliff park. A Mews helicopter will rescue you from drowning and bring you to the mansion to recover.
The process
Take the $5 bill from the tree at the right of Mansion Row, and buy a sports drink at the general store. Give it to the thirsty gardener and he will loan you his shears. These can be used to cut loose the balloonist at Cliff Park (far left) and get the rope you need for the surfing kite at Kitty's Kites. When you go back to Cliff Park and surf, Gretchen Grimlock burns your kite and leaves you to drown. Fortunately a MEWS copter lands to rescue you.
Instead of going right to the helicopter, go left when you leave. You can use the gate to Mansion Row, which is now open.
The Cryptids Museum is on the first floor of the Mews mansion, to the right.
When you are at any of the locations, use the helicopter. If you leave the island after getting the helicopter, you can use the gate at the far right of the town to leave or re-enter the mansion.
In the museum, first floor right in the Mews mansion.
When you come downstairs in the Mews mansion, and enter the museum at the right, your character will suggest watching the 5 videos on the cryptids. So you have to do that before Mews will appear again.
The upstairs room is never opened. There is nothing in there.
Go downstairs and to the right, to the Cryptids Museum in the mansion. Look at the 5 videos of cryptids, and Mews will return to give you some instructions.
After Mews leaves your room, drink the water and go downstairs, to the museum on the first floor (right). Watch the 5 videos and then Mews will arrive and you can begin hunting for the cryptids in his helicopter.
You awake in the Mews mansion. After he leaves, drink the water, go downstairs, and enter the museum (first floor right). Review all 5 of the cryptids videos and then Mews will join you.
The Cryptids Museum is inside the Mews mansion (first floor, right). You will review the videos there and bring evidence to the lab.
Since you do not return to the balloon (it leaves), the question may refer to the Poptropica blimp. To leave the island, fly back to the Mews mansion on the helicopter, then walk left past the house to the gate, and out to Mansion Row.
When you come downstairs in the Mews mansion, go to the museum (first floor right) and view all 5 videos about cryptids. Then Mews will appear to give you instructions.