I take it that you're refitting the bumper, or is are you upgrading from standard.
The reason I ask is that the side guides are different on the LCR bumpers.
I have just fitted LCR bumpers on my Leon, they can be a bit fiddly to slide home, but as the other chap said, if you remove the 2 top headlight bolts it makes the job easier.
You need to pull the bumper back towards the lights until it is flush to the wings then it should slide home easily. A bit of WD40 on the guides would make things a bit easier as well. HTH.