From Trondheim airport, board the train to Steinkjer. Link to trains
From Steinkjer to Sjøåsen, take the bus marked Namsos outside Steinkjer Station.
On the last part of your journey, you need to take public taxi (taxi for bus). Pay for it on the bus either by using debit or credit card, and it will wait for you at Sjøåsen upon arrival. The taxi will take you all the way to your destination, but it needs to be pre-booked (six hours before departure or on the day before, and of course within the taxis' opening hours).
To book a taxi call 07417 from Norway or +47 757 71 861 from abroad.