If you have a Tempo Disc or a Pebble, it will not be seen by your app until it is turned on.  For the Tempo Disc range of devices, the coloured surface acts as a button.  Squeeze the device between your thumb and forefinger to turn the device on.  There is a green LED that is visible behind the vents in the plastic on the bottom side of the device.  When pressing the coloured surface you should see the LED blink three times if the device is turned on.  If the device is already turned on, the green LED will blink once.

IMPORTANT:  Do not press the button with excessive force.  This has the potential of damaging the device.  If you do not see the LED blink when pressing the button the next step is to check the battery.