With iTunes open go to the menu bar top of your screen.
Click iTunes then click Preferences from the drop down menu.
Or, with iTunes open, the Command , (comma) keyboard shortcut to open the iTunes Preferences pane.
Preferences can be accessed on most apps from their menu bar. Example: Safari > Preferences
iTunes preferences aren't what you want anyway. iTunes doesn't actively scan for media on either Mac or Windows. Setting preferences for the media folder won't make it look at what is there already, it will just tell it where to store things from that point on. All those articles referring to preferences are telling you how to move media but the structured iTunes you see in the iTunes window is much more than just media. You want to move the whole iTunes library or else you will end up creating a new library on your Mac and lose all your playlists, ratings, play counts, date added.
Quick answer if you use iTunes' default preferences settings: Copy the entire iTunes folder (and in doing so all its subfolders and files) intact to the other drive. Open iTunes and immediately hold down the option (alt) key (shift on Windows) so you get a prompt to select a library, then guide it to the 'iTunes Library.itl' file in the moved iTunes folder.
In your case what you need to do is copy the iTunes folder on your PC to the external drive and start iTunes on the Mac as instructed above. However, if the external drive is formatted to NTFS for use with a PC this will not work with a Mac and you need to either install special software to enable this or you need to reformat the external for Mac use.