It should just push in and then the spring clip should effectively lock it into place. It can be a bit of a twat to get the shaft in far enough for the spring clip to engage though. When I fitted mine, it took a bit of force to get it to pop into place.

It may be worth trying a new clip. Also, though this is unlikely given that it's a Focus and using Focus parts, it might be worth checking the drive shaft plunge. So fit the shaft, but not the hub retaining nut. Then, with the suspension assembled, try pushing the shaft through the hub to fully compress the inner tripod joint. I don't know what the plunge figures are for a Focus to be honest but, if you loosen the inner boot, it should show you if the driveshaft looks stretched at all.