does dexterity only affect chance to crit for melee..or can you crit with charms as well?

No, dexterity only affects melée to my knowledge.

Dexterity: Increases your chance to critical hit, and damage dealt when you critical hit. It also reduces damage when enemies critical hit you. The effects are subject to a diminishing return formula.

So besides affecting the chance for a crit, it will also increase the damage of your (melee) critical hits.

Casters, however, can't crit. If you have X% Mind Numb on your gear, you can ignore the mobs intelligence, making your damage higher and it will sorta look like a casting crit

dexterity is just icing on a cake for a caster.

about all it will really do for them is reduce critical damage against themselves.