You should be able to pull the trigger down and into place through the trigger guard. Use a punch with a diameter 1/2 the size of the pin to align the trigger, locking block and slide stop, then insert the pin from the right side.
The slide should move without the guide rod/recoil spring in place, after you get the trigger pin back in place rack the slide, return it to the closed position, hold the take down catch down and pull the trigger, the slide should come off. Make sure you hold the gun with the barrel pointing down. You may need someone to help you by pushing the slide forward while you hold the catch down.