You have to go up through the layers of folders until you reach the very top one (you should see two folders: one for Poser 5 and one for downloads, if you didn't delete one), and then hit the "+" to add a new directory or library or runtime (I'm not looking at Poser 5 so I'm not sure what it's called but someone will surely know) and then it lets you browse for the runtime folder you want to point to. It's kind of confusing the first time but very easy after you get the hang of it.