Some stations have the ability to add additional sensors using channels. To confirm your station has this ability consult the product manual here.
How to select a channel for the outdoor sensor:
It is best to confirm how to select a channel by reviewing the product manual for your station as every station isn't the same.
In general, however, for most temperature/humidity (TX141TH-BCH series sensors) make sure to select channel 2 or 3 on the actual sensor. This switch is located inside the battery cover.
To view add-on sensors (in general):
Press the sensor button to toggle through readings from sensors 1, 2, or 3. When multiple sensors are connected you can view individual sensors or auto-scroll through all three channels. The auto-scroll indicator will be present on the display and switch to the next channel about every 5 seconds.