You need to make settings on the printer to use the Booklet Finisher.
When copying
Tap Settings > General Settings > Printer Settings > Output: Copy, and then select Finisher tray.
When faxing
Tap Settings > General Settings > Printer Settings > Output: Fax, and then select Output tray or Finisher tray.
When printing from a computer, memory device, and so on.
Tap Settings > General Settings > Printer Settings > Output: Other, and then select Finisher tray.
When printing from a computer, you can specify whether or not to use the finisher unit from the Output Tray menu on the printer driver.
When using the finisher tray, do not remove your printouts while the print job is still in progress. The print position may be misaligned and the staple position may move from its original position.