The SRM system has three different 'sensors' on the bike (i.e., exclusive of HR):
1. the speed sensor;
2. the cadence sensor; and,
3. the power sensor.
The PowerMeter includes the cadence sensor and the power sensor in its housing, but speed requires a separate wireless transmitter, typically attached to the chainstay with zip-ties. The Duotap negates the requirement for the speed sensor, as it broadcasts speed data in ANT+.
In order to 'activate' the PowerMeter cadence sensor and, as Karsten mentioned, the reed switch for power, the PowerMeter requires a separate magnet positioned within the arc of the reed switch. Personally, I do not recommend using the stock SRM magnet for this purpose, as it is more intrusive than required. Instead, just glue a 1/2"x1/8" rare earth magnet to the chainstay/bottom bracket shell (depending where the reed switch is in relation to the frameset. If I recall, on the Madone, we put it high on the chainstay, as it was flatter there).