"EM energy" is light. I.e., photosynthesis. You might be able to get useful energy from the radio noise generated by charged particles captured in the plane's magnetosphere, but your plants would have to be pretty big to develop biological radio rectennae. And they would need very highly developed ion-pumping mechanisms to capture and save up teensy bits of energy from each radio photon in order to add it all together to power any single useful reaction.
You cannot extract energy from a magnetic field without destroying that magnetic field. If you want to generate energy using the planet's magnetic field, the only way to do that is for the organisms in question to move very quickly across the magnetic field lines, and then harvest their kinetic energy via magnetic induction. In order to get biologically-useful voltages out of that, your magnetosynthetic plants would have to be very large, even more so than radio-eating plants, and therefore most likely multicellular. It is perhaps possible that such plants could live in cyclones or high-speed circulation bands such that winds push them across the magnetic field lines, and that's where the energy ultimately comes from, but it seems to me that this would be a rather marginal energy-extraction strategy; you'd do better relying on static electricity in storm clouds, or thermal gradients between different layers of the atmosphere.