If you mean the new built-in installer...it's very easy. You log in with your Renderosity ID and password, and it will show you the items you've purchased. You can download and install them from right in Poser.

I don't like it because you can't put things into different runtimes; they all go into one runtime. However, the old way of installing things still works. You don't have to use the new installer. If you prefer, you can do it exactly the way you did it with Poser 11.