Once you have returned the tickets to the man who lost them, use the one he gives you.
(see related question)
In the bin near the tunnel entrance.
The tickets to the Underground Tour on Counterfeit Island are at the bottom of the round garbage can to the right of the entrance. Once you have them, you have to return them to their owner, who is at the Web Browser Cafe on main Street.
you have to take the underground tour
go in the underground tour
There are two in one of the round trash cans to the right of the Underground entrance. You have to dig through the trash. (see related question)
You have to get a job at the museum after you take the underground tour
A green beret is worn by the Tour Guide on the Underground Tour on Counterfeit Island.
You can look around the island all you want. The "Underground Tunnel Tour" requires tickets. (see related question)
On Counterfeit Island in Poptropica, the tour tickets can be found in the bank. Head to the bank and look for a ticket booth on the right side. You need to interact with the booth to receive the tour tickets that will allow you to join the counterfeit money tour.
It is between the Underground Tour and the Police Station in Downtown.
The can with the tickets is the round one just to the right of the Underground Tour entrance.
It is about halfway through the Underground Tour, and you can climb up the wire from the light bulb.