Based on the message, it sounds like you have a OEM version of PD8, like PDR-HE, PDR-DE or something. What PD8 version are you using? The full version of PD8 (Deluxe and Ultra) both support Dolby 5.1. If you want to use your version of PD8, your best option would be to use a video conversion utility and change the audio to LPCM and then edit that file in your version of PD8.
Jeff