http://www.daz3d.com/products/daz-studio/daz-studio-what-is-daz-studio/ is the place where you download the product that you have specified with that reference. There should be a read me within that zip with a serial number in it. It Does appear you can also download it through your account here, which should then put it in your account, in available downloads, with a serial number in your availbable serial numbers, also found in your account dashboard.
As it states in the thread Jaderail linked you to, there is a serial number for DS4.5 in the top post, shown right at the bottom of the first post, if you donwloaded the RC3 from the link in that thread, which again puts it in your cart, so you need to complete the purchase.