Trouble with plastic piece for new radiator

[floridaman407]

I don’t know what is the issue. I had to replace the radiator on my 2008 G35. I thought i installed the plastic piece correctly but the coolant is leaking out of the plastic end where the red arrow is. I have tried to look for videos everywhere on how to properly install it in case i did it wrong. Every time i do see a video where they have the same radiator and plastic piece, they never demonstrate on the camera on how to install it. Any help would be greatly appreciated.