Unsulphured blackstrip is what you're looking for, but any thick and dark molasses should do. This is my first time using it, so I don't really have anything to compare to, though I have read that yield can increase by as much as 20%.
I don't know what kind of system you're using (soil, hydro etc.). I'm growing using indoor hydro and for that application you should start adding 15ml per gallon of reservoir water, in addition to your normal fert(s) as soon as your plants start budding. You should be replacing your nute solution once per week.
Lastly, you might want to do a forum search before asking a question next time. There have been lots of these threads lately.