By address

local component = require("component")
local emp = component.proxy("xxxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx")

By searching through all the components

local component = require("component")
local emp = nil

for k,v in component.list() do
  local p = component.proxy(k)
  if not (p.isICBM == nil) and p.isICBM("component_emp_tower") then
    emp = p
  end
end

• emp.getRadius() : number
  Returns the current radius setting of the EMP Tower.

• emp.setRadius(r: number)
  Sets the radius of the EMP Tower.

• emp.isReady() : boolean
  Returns true if the EMP is ready to fire an EMP, false otherwise.

• emp.launch() : boolean
  Will activiate the EMP tower if the EMP is ready.
  Returns true if it successfully fired, false otherwise.

• emp.getMode():number
  Gets the mode of the EMP Tower,
  0 = Both, 1 = Missile, 2 = Energy

• emp.setMode(mode : number)
  Sets the mode of the EMP Tower,
  0 = Both, 1 = Missile, 2 = Energy

• Getting Started
• Example Programs
• Radar Documentation
• EMP Tower Documentation
• Missile Launcher Documentation
• Cruise Launcher Documentation