Paring the Heart Rate monitor with the Oregon 400t is relatively easy. On the Oregon 400t select Setup -> Fitness -> Heart Rate monitor.
Once that is done, the Oregon 400t will search for the Heart Rate Monitor. In order for the paring to work properly for the first time, the Heart Rate monitor must be don by wetting some water on the inner side of the sensor which is on the left and right of the strap. After that is done, there is no need to pair both units in future again.The Receiver will show reading only if your body had some moisture so do not worry if you are not seeing any reading.
In order for to see the "bpm" readings on the Map mode on your Oregon 400t. You will have to go Setup-> Map -> Show Data Fields Always -> Always.
Go back to Map mode select any of the nav bar on the top and choose "Heart Rate"
There is no "ON" or "OFF" switch on the Heart Rate monitor, it is auto 'ON' once the receiver found the signal from the Heart Rate monitor but you must set it to 'ON'.
It is best that you either configure the Fitness Profile or create another Profile when use with the Heart Rate monitor. This will allow the Oregon 400t to search the Heart Rate monitor automatically at that profile. On the Oregon 400t Setup / Fitness, set the Heart Rate monitor to "ON". So every time when you switch the selected fitness profile, Oregon 400t will automatically search for the Heart Rate monitor.
The Heart Rate monitor weighs around 55gm
Battery type: CR2032