I have figured it out finally and this is apparantly all I need:
Application for Voluntary Membership in the Norwegian Social Security Program: https://www.nav.no/Om+NAV/Skjemaer/Privatpersoner/vedleggsveileder?entrance=classic&key=249734
Form N/USA 2 – ATTEST FOR UTSENDTE ARBEIDSTAKERE FRA NORGE TIL USA (CERTIFICATION FOR DETACHED
WORKERS FROM NORWAY TO U.S.A) – To be filled out by you and signed by work (and stamped) – Blankett USA NAV 2009
IMPORTANT – These should BOTH be sent to Nav International!!!! Do NOT send to Social Security Office in the US as it says on the form – this will only DELAY the process!