You don't open .prc files on your computer, they're for the Palm OS device, so what you need is to double click them to add them to the HotSync list, and then do a HotSync to install them on to your device.
(btw, if they don't install with a double click, try opening the HotSync Manager program and drag/dropping them into the window)
-THX
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Just because Windows users vastly outnumber Mac users, numbers alone do not denote a higher form of life. Cøckroaches outnumber humans many times over, and no one says a cøckroach is better than a human.