Oracle Text

Counter target activated ability from an artifact source. (Mana abilities can't be targeted.)

Card Rulings

10/4/2004 Can’t be used on mana abilities because they don’t use the stack. They’ve already resolved by the time you get priority.
6/8/2016 Activated abilities contain a colon. They’re generally written “[Cost]: [Effect].” Some keywords are activated abilities and will have colons in their reminder text.