Fortunately, you can benefit from the labor of others, as once a store is "unlocked," the Pumpkin Spice Latte will be available there for all. Each unlocked store will temporarily memorialize the occasion by displaying the unlocker's name on an in-store sign (in erasable chalk, I'm guessing).
So how do you get the secret code? Starting today through Monday, August 25, 2014, a daily clue will be tweeted by @TheRealPSL (the official Starbucks Pumpkin Spice Latte Twitter account, of course). In a collaborative effort, Fans can solve each clue and view their progress on Tumblr. Once all five clue/challenges are completed, fans/participants will gain access to a special password that can be used to "unlock" the Pumpkin Spice Latte by being shown or spoken to a barista at their local Starbucks.
Update 8/26/14 *spoiler* - the secret code to unlock the Pumpkin Spice Latte is "First PSL".