After installing the new paper tray on the printer. (Presumably, while the printer is turned off.) turn the printer on, then go to the Mac System preferences and select the "Printers & Scanners" icon. Next find your printer on the list of installed devices, and select it. Then select "Options & Supplies..."
When the sub-window opens, the top should have 3 tabs, "General, Options, and Supply Levels". Select "options." A new sub-window should appear.
On the "options" sub-window, you should be able to adjust the number of "optional lower trays" (or similar language for your particular printer). While this window is open, double check that the other configuration items are correctly shown (typically, memory configuration, feature set, etc.)
Once everythign is as you wish, click OK and then close the System Preferences window. The printer dialogs should then reflect how the printer is set up.
