Hi, yep you need to write reviews on the shopping site. Use the same email as you use here. When you see tester posts on the forum, read the instructions in the post and select what items you would be interested in testing (sometimes this is by a poll, sometimes this is by replying with the name of the item(s) but it will be made very obvious in the post).
The only other thing you need to do is make sure your postal address is saved on the shopping site.
The up to date info is all in this thread. That thread itself should be completely up to date but some other threads will have old information in them. For example, there is no longer a wishlist function and all testers are advertised on the forum.