First, I'd tap on the muzzle end of the barrel to ensure the receiver is fully seated rearward on the frame. Be sure to use ONLY a wooden, plastic, or rawhide mallet, so the mallet doesn't damage the barrel's delicate crown surface. Remember, you are just tapping, not trying to drive home the golden spike on the Intercontintental Railroad.
If your receiver is fully seated, then the receiver and the frame's bolt stop pin guide holes should be aligned and ready for the stop pin to be inserted. However, be sure to tilt the barrel fully downward (forward) to get the hammer spur pin to fall forward away from the recess for the bolt stop pin. Now push the stop pin into the hole, while the barrel is still tilted downward, and start it into the guide pin holes. You may have to apply a little force here to get the bolt stop pin to seat. If it still won't seat, then you may use the mallet to lightly seat the pin on any part of the mainspring housing that you can get a good surface to apply the force necessary to seat the pin. Again only lightly tap, do not hammer the pin in or you will break the bolt stop hinge pin.
If everything is aligned properly, then light force is all that will be necessary to seat the pin. If you don't get the pin to seat, then dissassemble the mainspring housing and visually check for any obstructions, like the hammer strut being in the way.
The first few times the bolt stop pin is inserted it may be a tight fit but after a few teardowns it'll go in a lot easier. You may also want to put a few drops of oil on the pin to help lubricate it as it passes through the guide holes.