I want to increase the number of push up's that I can do.  The gym I
use has Nautilus equipment.  Which machines should I concentrate my
efforts on.

NONE. There is no need for machines they will not help you increase
your push up amount quick enough for you to notice. You will end up
getting frustrated because your not doing any more sets than normal. I
was in the Royal Marines for 5 years and when I first signed up I
could do around 20 then id have to stop. Now I can easily do 200 in
one go. The process works by doing as many as you can til you
physically cannot do anymore, then the next day do the same, free
pressups no machines, if you have to then balance on your knees not
your feet. Also try varied hand positions. Shoulder width, arm pit
width, chest plate width and waist positions. But basically do them
without a machine and do as many as possible lifting your own body
weight. You will notice a huge improvement.

The above comment is great advice.  But if your gym also has free
weights, you may be able to take your pushups to the  next level.  If
you can lay flat on a bench and do chest presses with independent
dumbbells, this will help strenghthen all of the stabilizing muscles
in your chest.  Plus, if you can get to the point where you can lift
heavy dumbbells, it will make your pushups seem more like jumping
jacks.  To learn more about weight lifting go here:
http://www.shapeyou.com/

mattodd1 can you please explain further.

'free pressups no machines, if you have to then balance on your knees
not your feet. Also try varied hand positions. Shoulder width, arm pit
width, chest plate width and waist positions. But basically do them
without a machine and do as many as possible lifting your own body
weight. You will notice a huge improvement'

Thanks for the great advice.

Stuart