Well you are putting the pin in from the correct side, due to the spring on the left side. That a good start.
When I've put the trigger pin in, I press down on the locking block and then it pops right in. You should not have to drive that pin in with a hammer, if everything is lined up properly.
Try putting the pin through the HBI trigger outside of the pistol. You'll know then if it's a issue with the trigger. If so, I'd just open up the hole slightly.
Good luck with it and please do let us know the out come.
Best regards.