Selecting difficulty level with The Hero of Etheria?

Hi.  I'm new to The Protectors, or even Warlords Battlecry III for that matter, and I wanted to know if it's possible to select a difficulty level when starting a Campaign chapter.  I'm starting from the beginning with the original WB III campaign (The Hero of Etheria?) and there's no difficulty option when creating a new character.  Also, I noticed that when it sometimes lists a mission as "impossible" does this mean it can't be won?  One reason I'm using the Protector mod is that I got constant freezes with the vanilla GOG version (1.3?) using the widescreen beta 2 patch.  Hopefully that won't be a problem here.  I want to start as close to the vanilla game as possible but with all the enhancements and fixes Protector affords.

On the world map, go to menu->options and you can set the campaign difficulty there.

Impossible is just a rating, you can still win it.