You did remove the old one by inserting the punch into the hole in the top of the slide and pushing the pin out towards the bottom, right?
Same for installation, start from the bottom of the slide and (pound) push it upwards.
I get mine started first. Then put the spring/extractor in and hold it by my thumb. Then I tap the pin with the hammer to move it further towards the extractor. If I've got it lined up right the pin goes right on into the slide (usually doesn't happen that way though), if not I can hear/feel the pin hit the extractor and I push/shove with my thumb to reposition it and then tap the pin again. It may take some repositioning after each tap but you'll get there and the sound will change to let you know the pin is moving through the hole in the extractor.
Oh, one thing I do (probably unnecessary but makes me feel better) is to check prior to installation to make sure the pin can freely move through the hole in the extractor. Better to find out the hole is out of spec prior to installation vs. after getting frustrated during attempted installation.