Blue line coming from the center:

Go to the back of the train, go up the sliding path (sort of like an escalator, but flat), turn left down the corridor, up the escalator, go straight out of the metro station (still underground) through the sliding doors, then through the first set of glass doors, go past the Espresso House leaving it on your left hand side, go through the second set of glass doors, and up the escalator, turn left, go outside, turn right, continue until you see the Brewdog bar on your right.

Green line coming from the center (if you must):

Go to the back of the train, go up the escalator, go straight out of the metro station (still underground) through the sliding doors, then through the first set of glass doors, go past the Espresso House leaving it on your left hand side, go through the second set of glass doors, and up the escalator, turn left, go outside, turn right, continue until you see the Brewdog bar on your right.

If you do take the green line from the center, then you could get of at Sankt Eriksplan instead. I think this is the front of the train, and then from some exit, you should go away from the park, and across the bridge leading to Kungsholmen. The Brewdog bar is on the left side in this case.