It needs to be powered with RF. A Wireless Charger (from EnderIO) is probably the most convenient way to power it, but you should also be able to recharge it in a Thermal Expansion Energetic Infuser or (maybe) one of EIO's capacitor banks. If the electromagnet is literally the only RF-powered thing you intend to create in this world, however, you might be better off using Tuberous Flux Capacitors. They're basically single-use batteries made from potatoes, and while they don't have very much charge, the electromagnet doesn't use much, either. Put the capacitor on your hotbar and shift-right-click it, and it should power the magnet. The magnet itself may also need to be right-clicked or shift-clicked to turn it on.