The Frequency Transmitter are items that are used to link Teleporters. When right clicked on a teleporter with the Frequency Transmitter, it sets itself to the current frequency of that Teleporter. To link it to another Teleporter, right click on the destination Teleporter. Linked Teleporters effectively allow you to cover long distances instantly by using some EU power. After linking two Teleporters, it needs a reset of frequency or else it's going to link the previous starting Teleporter to whichever Teleporter you right click on. To reset right click anywhere.
This item is used to craft the Remote Thermal Monitor, an item used to remotely check the temperature of a Nuclear Reactor(and it's chambers), so is therefore a key item for the Nuclear Control mod. It is also used as a crafting material in all, or most, blocks/items in the Forcefield mod such as the Teleporter.