DO NOT pull it out!!!

Take him to the vet right away! He will need to get it taken out by the vet with sterile utensiles.

If someone had a knife in thier leg you wouldnt pull it out right?(or I hope not)

Apply pressure to the area (not where the nail is ) but around.
And take him to the vet right away!

They remove it and probly give you some antibiotics to fight any infection.

PLEASE DONT LET THE CAT AND RABBIT "PLAY" rabbits can be very nasty animals and they dont usually get along with other animals and when they dont like something they go in for the kill and right for the neck, belive me the rabbits teeth do worst damage then the cats. And the rabbit could probly kill your cat. If you were to ever introduce them its supposed to be in nuteral territory.
where both cat or bun havnt been. But because this happened it will have a life time effect on the bun and it will attack the cat. Please I speak from experience with rabbits and other rabbits/other animals being together. I had a baby who got attacked by another adult rabbit and had her throat split open by one, she could NEVER be put with another rabbit she would have ripped it apart.