You earn credits by being active on the site. You can earn credits in many ways, logging on, responding, liking post and several other ways. Points can be given away or used in the Shop to get additional site features.
You can check on your points here and see how you earned them.
When you redeem a subbox/featured video you will now have a new forum that you can access on the home page. In that forum you will see the forum for the option(s) you redeemed and you will then be able to make a post to share the link that you wish to use for that purchase. Only make one thread and you will get a response in that thread telling you when your video/subbox will be up. After your week is up you will no longer have access to that forum until you make another redemption.