The best way to dissolve it is to just use a little shot glass with hot water, put in the molasses stir for a few seconds and it is dissolved easily, then just add it to your nute mix. Note that the hot water will be in such a small amount that it will cool down immediately and won't effect the temp of the nutrition solution.
Some folks actually boil the water, I just use a shot glass of hot tap water to mix the molasses and add it to my nute solution with RO water. This is much easier than boiling water each time, and works flawlessly.