I had one on my G19. The kit should've come with a tool that looks like a mini-punch. You would need a needle nose plier or a pair of tweezers to hold the spring and push it in with the tool. Hope that helps. Do not use it without using their spring. I tried doing that cause I had difficulty installing it at first. I left in the Glock spring. It caused the laser to go on everytime the gun got jarred. The spring is stiff for a reason. It holds the on/off switch in place.  

Edited to add: I loved the LaserMax. Usefull tool. I would point at an object then turn it on to see if I hit it. Also has the intimidation and cool factor to it.