Nirvana in the sense of freedom from Ignorance that comes by realisation of the Divine within through the psychic door is indeed the first step of Integral Yoga. While Sri Aurobindo described his experience as Nirvana, the word is generally not used in this yoga because it has a different connotation in traditional spiritual literature that usually implies other-worldliness and a cessation from the cycle of birth and death. It also implies to many a cessation from works or a complete inner stillness and immobility that generally comes through withdrawal from life and activities into the trance of Samadhi. Therefore when Nirvana is posited as the first goal, the general understanding and tendency is to first withdraw from life and activity until we enter into immobility and stillness. Later one can return back into life and speak about the Divine Manifestation or transformation.
While this may be the course chosen by some souls that are drawn towards the ideal of Nirvana, it is not the generally preferred course for the yoga of transformation. This is so because the push towards Nirvana generally takes one farther and farther from the fields of life and action. Nature is left unprepared in the process except for some degree of sattwic modifications that would not impede the pursuit of silence. But for this very reason it becomes difficult to return and work upon these unprepared fields and activities.
A better and the recommended way is to arrive at freedom from Ignorance (Nirvana or Mukti) is by undertaking all activities in the spirit of the Gita. Here the insistence is on Peace, calm, equanimity that come through the practice of nishkama karma (desireless action) undertaken in the spirit of service to the Divine and dedicated to the fulfilment of the Divine Will in our own life and the collectivity. This practice strengthened by an inner Remembrance of the Divine Mother gives us the same result as Nirvana in a much easier way. Besides since one has not withdrawn from life and activity, there is in this process a simultaneous ongoing preparation of life and nature to receive the Divine and Her transforming forces in it.
If one pursues this way, as is recommended by the Mother and Sri Aurobindo, at a point of time the psychic being emerges from behind the veils of our surface nature and we begin to grow conscious of the Divine Mother’s Presence within us. Then the process of transformation begins in the real sense and in an increasingly purposeful and intense way until Her Force invades and takes hold of all the fields of nature, opens all that is closed, demolishes all that resists and retards and eventually shapes us into a pure and perfect channel and wide and plastic vessel, a strong and supple vehicle and instrument for Her purposes in the world.