Each year stands on its own. You should not be reporting 2006 income on the partnership's 2012 tax return. You should, however, restate the opening balance sheet to reflect the previously earned income. It is too late to amend 2006 as the year is "closed" (being more than 3 years old. I am assuming that the omission was not intentional and merely an oversight. If I am wrong, you should seek tax counsel. Good luck.
Marty Davidoff, firstname.lastname@example.org, 732-274-1600. This answer is provided for general information only. You should seek advice from an attorney or tax professional.