So, Open Poser and click File > Install from Zip Archive.
Navigate to your zip file, likely in your download folder.
Click the zip and then click Open.
Poser will offer a screen to let you choose where you want to install your new purchase
Choose your runtime
I chose to name mine P11 Content on install.
You should see two options at the drop-down, Downloads and whatever your Poser 11 default runtime is named. Either will work, but it would probably be easiest to use the default runtime.
The open field below the existing runtimes is for you to create a new runtime on the fly. I do not know if poser will map this for you, or if you will have to do that yourself. If you don't know how to map a new runtime, avoid that field! ;) Click install and your files will be correctly placed.

;>

Andy!