Enlightenment comes in large part due to removing the afflictive emotions that obscure our Buddha nature. The Buddha nature itself is eternally present, but that does not mean that we are already Buddhas (at least according to what I have heard from my mostly Gelug teachers, and a teacher from the Karma Kamtsang Kagyu tradition as well).
The nature of the mind is pure, the stains are adventitious, but that does not mean that we do not have to strive to remove these stains. So in fact, while we have the Buddha nature, we do need to do something to "sad" tibetan for "activate it.
Comments on The Sublime Continuum treatise on Buddha Nature from Chokyi Gyaltsen "Ocean of Sport". Translation Jampa Gendun
The meaning [of the second simile is as follows]: when the element [of buddha] –
the sugata essence – in the continua of all sentient beings is freed of the stains of
obscurations, there is a way that it transforms into a nature body. The great sages,
seeing with their eye of exalted wisdom, the omniscience of the buddhas, that the
element [of buddha] – the sugata essence – in the continua of all sentient beings is
enclosed within the covering of the afflictions, see that in dependence upon
instructing in the methods that free from those afflictions, when it is free from all
the adventitious stains of them, it becomes a nature body. Analogously, a person
trying to get at honey surrounded by a swarm of bees clears away the bees with a
method; in doing so, the honey is accordingly made manifest.
In teaching the third simile Sublime Continuum says:
Just as the kernel of unhusked grain
Is not what people refine;
So those who want food and so forth
Extract [the kernel] from its husk. [v.1.107]
Similarly, as long as the victor within sentient beings
Remains mixed with the stains of afflictions
And is not separated out from them,
That long will it not enact a victor’s deeds in the three realms.
In order to ensure my mind never comes under the power of the self-cherishing attitude,
I must obtain control over my own mind.
Therefore, amongst all empowerments, the empowerment that gives me control over my mind is the best,
and I have received the most profound empowerment with this teaching.
brtsal ba'i bkhra drin