I'm current using the Co8 mod pack and the increase in weapons and NPC mod is great. However, I stay with using great maul for my fig/barb. But for some reason I can't specialize in it. Whenever I choose weapon focus, I can't find the option for maul. Anyone else experiencing this problem?

Is great maul listed as a proficiency? From my knowledge of the 3.5 rules, it's much more likely to be a greatclub, which is an actual weapon proficiency - if I'm right, specialising in greatclub should apply the bonus to your great maul.

Actually I solved shift + click on the great maul and apparently, it is classified under warhammer along with warhammer and great warhammer.