You don't say what OS you're using, but newer versions of Windows are paranoid about putting files in the "Program" folder. Because of that, don't put your runtimes in the "Poser" folder. Just too much of a pain. (Poser's default runtime now installs in the Documents folder.)

Aside from that, it's pretty easy. When you open the zip file, there should be a folder called "Runtime." Just extract that into the folder you want to contain the runtime. For example, I have separate runtimes for each figure, so I have folders named Vicky4, Mike4, LaFemme, Dawn, etc. Each of those contains a folder called Runtime. If I buy a new dress for, say, LaFemme, I'll extract the runtime folder into the folder called LaFemme. Everything should fall into the right directory structure automatically.