The winnings from a free spin bonus are converted into a new bonus that must meet certain wagering requirements. Because their weren't any requirements with the free spin bonus, the winnings must be played as a no deposit bonus before any funds can be withdrawn from the casino. Generally, you can take this new bonus and play the slots until you've met the wagering requirements and then cashout your winnings.
Once you've met the wagering requirements, then you'll be able to request a payment within the casino cashier. You should check out the wagering requirements for the standard no deposit bonus within the casino to understand what must be met before you can withdraw your winnings. Also, their may be other requirements in order to withdraw your winnings. Some of the casinos will require a minimum deposit first.