I just finished painting my front door with Kelly-Moore Epoxy Enamel
using a HVLP gun. The un-thinned paint was very thick - on the order
of pancake batter. Initially, as you did, I used the thinning
recommendations on the can and got an orange peel finish like you. I
have found that to get latex to spray correctly you need to do the
following:

1) Use a larger fluid nozzle and needle. My gun recommends a 2.0 mm
nozzle for latex. This is what I used.

2) Use Floetrol as recommended by others.

3) The following is the most important. Forget the thinning
recommendations on the can. Get a viscosity cup and thin the paint to
around 22 seconds. You will be surprised how thin a 22 second
viscosity is and how much water you will be adding. The final
viscosity will be on the order of milk. If you do not thin the paint
it will not flow out on the surface, and you will have orange peel.

When you spray make sure you get a wet surface but don't lay the paint
on so thick it will run. If the coat is not wet in will not flow out
and you will have a sandy surface. Remember the thinned paint is much
more likely to run. Do not try to get one-coat coverage with the
thinned paint. Your first coat will make you think you are spraying
water and look more like a stain rather than paint. It will take
several coats to get a nice buildup but the thin coats dry quickly,
and several coats can be applied fairly quickly. Watch the spray gun
tip. I got a rapid buildup of dried paint around the nozzle.
Eventually the buildup effects the spray pattern or worse breaks free
and lands on you work. I just kept a bowl of water and a toothbrush
nearby and when the buildup got too large I would hit it with a wet
toothbrush. The thinned paint tends to separate quickly so make sure
you keep it stirred.

Using the above procedure I got a near glass-like surface on my door.

Good Luck