On the bottom on the game screen is the progress bar. To the left of this bar is a plus sign beside a monkey bucks symbol and a hammer symbol. Clicking on the the "plus monkey bucks" symbol takes you to a screen that lets you spend Facebook Credits on the in game currency, monkey bucks. This currency can then be used to purchase items in the game such as more hammers.

@Crystal - Please try refreshing your browser first to see if that will help. If the monkey bucks do not appear in your inventory after that please send in a support ticket by pressing the Help button either in the top left corner of the game screen or the help button above the game screen. Those buttons will lead to an "Open Ticket" button were you can describe the issue.