Canon Rebel T2i Quick Start Manual page 112

Establishing Wireless Transmission
The Sennheiser AVX wireless clip-on microphone
must be plugged into a transmitter.
Turn on the transmitter first and then turn on the
receiver once it is plugged into the WAVE recorder
Transmitter
and has phantom power (see next slide).
Then long press the PAIR button on the transmitter
until it flashes green. Then long press the PAIR button
on the receiver until it flashes green. A link will be
established shortly and both lights will be green.
Receiver