To print to Mailbox, you need to ensure you are using the Canon PS Driver for OS X, preferably v1.70 or later. There is also a Canon UFR2 driver for OS X, but it is not available to all models of imageRUNNER.
With the correct driver installed, when you select to print from an application, change the Copies and Pages menu to Special Features. Here there is an option called Job Processing and in this menu you have the options of Print, Store, Secured Print and Promote Print. To print to mailbox, select the Store option. Then when you press Print the window letting you select the Mailbox number and enter the file name will appear.
If you don't have the Special Features tab but have installed the PS driver v1.70 or later, then something has gone wrong with the installation and the driver will need to be installed again. Please reply if you need help with this.