Make sure you are using the correct wiring scheme on the CAT-5 cable:http://en.wikipedia.org/wiki/Power_over_Ethernet#Power_capacity_limits
I usually use T568B wiring on straight cables and it works OK. If you wire a T568A on one end and a T568B connector on the other it will make a "crossover" cable, saving you the need to buy a switch to interconnect 2 devices.
Try using a short 6' jumper as a test. If it works, there's your answer. Also, it could be that the POE feature is not true POE and doesn't like the 48VDC or so that the trendnet injector is putting out. If that's the case, there are passive injectors available, but I don't know that 5VDC would survive the 100' trip out to the antenna.