Open your P5 library and click the "up" arrow until you reach the uppermost level, you see a Poser 5 folder with a red dot on it (that is, yes you guessed it, your Poser 5 runtime) and a "Downloads" folder (that is a folder Curious Labs has created already as an extra runtime in the Poser directory). The red dot indicates your currently active runtime. Scroll down and you wull see a small "plus" sign. Click on it, and you are prompted to locate your new runtime on your disk. Once you have done that, it will appear with the other two folders above, and you can select it as your active runtime at any time.