The AP1252 has higher power requirements than some of the Cisco PoE switches provide.  Depending on the switch you are plugging them into you will get enough power to power up the device and connect to a controller but not enough to power up the antennas.  Options that can work include:

1) Removing one of the antennas if you are not gonig to use it (or as a temporary workaround to get some wireless)

2) Using an external power supply

3) Using an external inline power appliance system that supplies enough power (http://www.microsemi.com/powerdsine/ is one example).

4) Depending on the type of switch you are connecting to you may be able to upgrade code to provide a higher amount of power to the port.  For instance, a Catalyst 6500 by default supplies 15.8 watts of power to a port.  But if you upgrade the code to 12.2(33)SXH2? I believe you can get 16.8 watts of power and this is enough to bring up both antennas on the AP.  But even then you don't have enuf power to run the upper N speeds - I think anything above 72 Mbps is unattainable.

Other AP models such as the 1140 have lower power requirements and may work better for you.