Give me the image below, and I'll write it down for you. Everything you've above is right and it should work.
local screenW, screenH = guiGetScreenSize()
addEventHandler("onClientRender", root, function()
local zombieKills = tonumber(getElementData(localPlayer, "Zombie kills")) or 'None'
dxDrawRectangle(screenW * 0.0454, screenH * 0.6263, screenW * 0.1691, screenH * 0.0404, tocolor(0, 0, 0, 155), false)
dxDrawText("Zombie Kills: ".. zombieKills .."", screenW * 0.0527, screenH * 0.6406, screenW * 0.2072, screenH * 0.6536, tocolor(255, 255, 255, 255), 1.20, "clear", "center", "center