You redeem your own credit by using your own created promotion code. Under your profile you will see an area for you to enter your own promotion code. Do that.
Then go ahead to register for any course and on the buy page where it says "if you have a promotion code from a sailing school or charter company click here" like the image below - then click that and enter your code there. The appropriate credit will be applied.
To check your credit, click on your Gifts and how to get free courses tab at the bottom left after you sign-in.
Then click on the credits tab
There you will see your credits