Depending on the age of the OS the Mac allows merging, however, the computer gods may not be friendly on the day you do it and will send them elsewhere. I manually open the zips > then go to the inside folders to runtime >  and go one folder at a time, putting the items within into the exact matching folder on the Poser folder where the runtime is located...or if you have an external runtime then again match the folders with the same one on the Poser program..

I forgot that grabbing items from the free section does not show up in your account, sorry about that.