Open your web browser, type this exactly "about:debug", without the quotation marks in the url bar and hit the go or search button. About debug locks the web browser and adds a debug category to the settings menu. Now hit your menu key then settings, now scroll down until you see the newly added debug section. Select UAString and now select desktop or pc. FYI You will also see other strings like iPhone or iPad if your phone has it. What UAstring is suppose to do is make certain sites think your phone is something other than Android. Like Hulu and Google are still having issues, so Hulu has blocked Android phone from its site, so you set your UAstring as an iPhone and your in. Hulu caught on to that trick so it won't work with them, but you get the picture. Now blackout of the settings menu and type in the site you want to go to in the url bar. The web page will unlock itself and you good to go. I just leave my Droid on desktop mode and it seem to work fine. Good luck.
If you're trying to enable Flash (the software), then you'll need to do the about:debug thing in the browser on the phone. If you're trying to flash a ROM/Kernel/etc, just plug the phone into the computer, drag down the notification bar, click on USB connected, click on connect USB (or enable, can't remember the exact phrase on the button), and you should have a new hard drive on your computer to correspond with your SD card.
However, it still depends on what you're exactly trying to do.
To do that I believe you need to be in diagnostic mode, which means you need to hold T down while you boot up. I assume that is what you are referring to as PC mode.
You really should find a decent guide or a better "guy" before blindly going into this. If he missed the step on how to boot into diagnostic mode, more than likely he missed other important information too.