In late 2009, Northwest Airlines began its merge with Delta airlines. Northwest Airline's frequent flier program known as "WorldPerks" merged into Delta's frequent flier program known as "SkyMiles". Those who were part of the Northwest "WorldPerks" program should have automatically had the points transferred into a new "SkyMiles" account with Delta airlines in late 2009 or early 2010. However, if you're unsure whether your miles were transferred correctly refer to Delta's website for further instruction.
Go to "Delta.com" and roll your mouse over "Need Help" which is at the top, just to the right of the center of the website. A dropbox will come down.
Scroll down the drop box and click on "SkyMiles". A window with the title "Contact SkyMiles" will come up.
Call the phone number on the website or e-mail the company. They will be able to give you information as far as where your Northwest miles have gone and what account number they're under.