For a long, when I did my first task on mpesa payments, this was perhaps my hardest part of it. The developer account for Safaricom has almost every tutorial and items you'll need during this process; unfortunately this information is not there on their website.
After long googling and online searches with no avail, I swallowed my pride and emailed them, and Hurray, that was the solution. If you need a passkey for mpesa online payments solutions (specifically express stk push) simply send an email to email@example.com and they will send it back to you.
First, you will need to have gone live with your app on their portal, provide them the app details, they will approve the app and send back with passkey details. The mpesa guys are so first on this, actually same day feedback.