These devices would be added the same way that any other wireless device is added as a client to a network......
1) Scan to select the network
2) Enter the wireless network password
Keep in mind that DirecTV cannot connect to a wireless network if there are special characters.....like an apostrophe, asterisk, dollar sign, etc......in the wireless network name.
So if your wireless network is named something like Ernestomelia's Wireless Network, DirecTV will not be able to to connect. Instead, the network should read something like ernestomelia network. A shorter name with no spaces would be better.
Your printer might have similar requirements, but you did not provide the make or model number of the device....so we don't have much to go on here. In general, you would want to contact support at the printer manufacturer for more specific information.
It is not the device that should show up when you scan. It is your wireless network that should show up when you scan using the DirecTV DVR or wireless printer.
The AirPort Exreme does not support a PIN methodl of connection. You will need to connect the printer and DVR using a manual setup method. That may require that you contact support for DirecTV and/or your printer.