Sideshow is one of the first exclusively online toy and collectable direct sale retailers, They got burned by the major retailers and went online exclusive because of it. One way to insure direct business at their site is to offer Sideshow.com online exclusives, they make a bit more money that way. A larger number of exclusives would hurt sales of the figures from other affiliate retailers, which is another important part of their business. Also, there needs to be some incentive to become a newsletter subscriber. And I personally like the idea that online toy news websites wont be allowed to cheat the loyal newsletter subscribers out of their exclusive.
If you get this weeks newsletter and set up your account it should be fairly easy.
The way it goes is I think on Friday at the time specified, use your new login to Sideshow's website to see the Newsletter, click on the preorder, place your order. You may have to login to your main account too.
I could be wrong about the specifics, I haven't set my account up just yet.
If your not receiving the Newsletter, maybe it would be best to call Sideshow Direct and ask them to send the newsletter to your email, or perhaps they'll help you set up your account over the phone.