Complete the last five levels in the Las Vegas level. Then, on the Grand Finale, choose the stretch limos for the thing to jump on. Make sure the number of them is on maximum, and that your settings to the first three are very good - leave the bottom two alone. Make the jump, and you will hit an invisible ramp above the real one.
Reset Your Game
Enter: RESET as a password.
Snake River Canyon
To unlock the fifth set of stages, the Snake River Canyon, you must beat all five stages in Las Vegas, Los Angeles, London, and San Francisco, then return to the stage selection screen to find the new option in the middle of the four.