if it's the thumbtack style, just push with the end of your thumb. if it's a standard tack, use a small hammer, like a ball peen.

since you only have one tack, a tack gun would be a waste of money.

now if we're talking about a TACH, or tachometer, the green wire is the rpm monitor, and should be tapped into any rpm signal lead. the easiest one is on your ECM.