I'll describe the process I went through, I am sure Rich can add more info

Start here:
http://motorcitysynthetics.myamsoil.com/Click on SHOP ONLINE. That takes you to the AMSOIL site, with MotorCity's referral code in the URL.
If you want discounted pricing, buy the 6 month or 1 year membership (that's the only thing you buy the first time). You will set up your account (ID/Password) during this process.
Wait a few days to get the membership packet in the mail (< 1 week usually). Once the membership is active, your discount should show automatically when you log in and place items in the cart. I would check daily to see if this active, in advance of receiving the paper materials.
Check to see if you are near a distribution center if you want to save on shipping costs. I am a bit far from the 2 centers closest to me, and am better off having the items shipped to my home directly.
Select the items you wish to buy and add to cart.
Place your order.