how can I connect a Canon MX892 wireless printer to my airport extreme

The AirPorts support WPS, but only in a very limited case, and typically only works with other Apple products only. With that said, if you would like to give it a try, the following are the basic steps to do so.

1. Open the AirPort Utility
2. Double-click on your AirPort base station icon in the utility, and then, enter the base station password if prompted.
3. From the Base Station menu, chosse Add Wireless Client. This will open the Wireless Client Setup Assistant.
4. There are two types of WPS connections, PBC and PIN. Choose the one that your printer support.

If your printer supports the PBC connection method:

1. In AirPort Utility's Wireless Client Setup Assistant, select the First attempt option.
2. Click Continue.
3. Push the WPS button on the printer. You should see the MAC Address of the printer appear in the AirPort Utility Wireless Client Setup Assistant.
4. Click Done.

If your printer supports the PIN connection method:

1. In AirPort Utility Wireless Client Setup Assistant, select the PIN option.
2. Click Continue.
3. Enter the PIN number that is either hard-coded in your printer or displayed on the control panel of the printer.
4. Open the setup assistant application installed with the printer software to configure the printer for your wireless network.
5. During the setup assistant process there should be a step in which you will be asked to choose your wireless network. This will be the name of the wireless network that you noted earlier. If your wireless network is password protected, you will be asked to enter the wireless network password that you noted earlier.
6. Once the process is completed, your printer may restart to join your wireless network.
7. Open the Print & Fax preference pane in System Prefereneces, then add the printer via the "+" button.

Ref: Apple Support article