Metal doors are a little different. They don't take so well to trying to use a chisel to mortise out a latch plate.
Some are already prepped for a deadbolt and have the tabs welded in place for mounting the latch. Most are not so now comes the fun part.
The easy way out is to use a drive in latch and hope the deadbolt never gets a little loose so the bolt never slips out of the end of the door. Some are better than others.
You can use a product called a bridge that looks like this
You cut out a rectangular hole in the end of the door to match the bolt. Insert this into the hole and pop rivet it in place. This gives you something to screw the latch plate to.
Another trick to these is to use a Squeeze Play to form a rectangular dent (if you will) into the end of the metal door to mount the bolt. These are very expensive but work extremely well.
Another trick is to use a door reinforcer plate that has a place recessed into the end of it for mounting a latch plate.
Something else to watch out for on this hollow metal doors (especially residential grade) is there is nothing to keep the door from physically collapsing when you tighten the screws that hold the lock together. You can purchase inserts from various mfg's to prevent this from happening. You simply drill the cross bore holes, insert the sleeve and then the lock.
http://www.hudsonlock.com/ESP/ames_door_lock_support.html
Here's a pic of the template similar to what I use for marking the holes
There are still special cases that will arise when working with metal doors like having a seam down the center of the edge of the door. They make special mounting bridges for these also.
Any questions, please feel free to ask.