http://iscoresports.com/

How do you score this in iScore:
(softball) Runner on 3rd, Full count
Pitcher throws a drop ball that hits the front of the plate, batter swings (and misses), catcher drops and blocks; catcher throws ball to first baseman, ball hits first baseman's glove (in the pocket) and ball is dropped. Runner at 1st is safe and runner from 3rd is safe at home.
I can't figure out how to assign the error to the 1st baseman.

From MLB rules:

Rule 10.12
(d) The official scorer shall not charge an error against:
(5) any fielder when a wild pitch or passed ball is scored.

(e) The official scorer shall not charge an error when the batter is awarded first base on four called balls, when the batter is awarded first base when touched by a pitched ball, or when the batter reaches first base as the result of a wild pitch or passed ball.

The only time you would give an error would be if the runner made it to second as the result of the throw... then you would be given the chance to say the batter-runner made it to second as the result of an error. But they made it to first as the result of the wild pitch or dropped ball, and there is no error even in the case of a wild throw or dropped throw.