For those asking and have the game on Steam, if you purchased the game via the KSP store prior to the free DLC cut off date, you should find that your steam version of KSP is called 'Early Adopters Program' or something. I can't remember where it refers to it as this, I think in the properties window, last tab, but this should indicate you have a version that will automatically receive the DLC.
It also should not matter when you activated the game on Steam, as long as it was via the linking process within the KSP store that was available at the time.
I personally was getting worried until I found out about the above as I no longer have access to my original KSP store account (email address associated with account is no longer under my control).