I am past level 21 but there still isnt an extra slot for my off hand weapon.
is there anything that i need to do to unlock it?
It’s actually based on item level right now, so you need to find a level 21 weapon or above.
Also, this is going to be changing for the next patch, unlocking the skills at character level 10 and 20 instead.
i just double check, i have more than level 21 and 26 offhand weapon but i cant equip them too… i just can equip one offhand weapon 
To clarify 
For any weapon to be able to use their 2nd skill, it must be item level 21.
For any offhand to be able to use their 2nd skill, it must be item level 26.
Sorry for the confusion!