Keyframing is enabled always. Do you mean the auto keyframe button?
If so, what version of AE are you using? It was moved to a menu in a recent version of CC. You can enter auto-keyframe mode by choosing Enable Auto-keyframe from the Timeline panel menu.
Are you referring to the stopwatch that is next to every property name? If that's not visible, then you have probably just hidden the column that contains it, so you just need to show the column:
If you are looking for the Auto Keyframe switch in the top bar of the timeline, that feature was removed. Otherwise, reveal the property you want to animate as Todd said.