When you turn on the keyboard,the blue LED Will stay on,that means the keyboard has paired with your device.

If the Blue LED does not stay on and keeps flashing,it means the keyboard was not paired with your device.As for how to pair,please try following steps:
Press ESC button and turn on the keyboard at the same time,then release the ESC button and the BLue LED will flash fast.
Plug into the USB dongle,the flashing blue LED will become solid on once paired with success within 60 seconds