You need to be logged into the account you purchased the movie with. You can check and see if it's your current account by going to your
My Stuff and looking at your "DM Threads I Own." That has a list of all the videos you've purchased or were given access to, and you can click the links there to take you to the thread where the video is.
The video is usually embedded at the bottom of the first post of the sales thread, so you might need to scroll a bit.
Another problem that sometimes occurs is that sometimes people purchase something on permies while logged out and use a paypal account that's linked to a different email address. When that happens, permies autogenerates an account linked to that paypal email address. You would need to be logged into the that other account to access it. If that happened to you, you can post here from both accounts and we can merge the accounts together.