Where is the SKY PILLAR in emerald?

To the right of Pacfifc Log Town.

First you need to beat the elite 4 the n get a mach bike. Fly to pacifilog town next. Surf over the the right then you will find an opening. Don't forget to bring tons of ultra balls (or use your master ball if you still have it) and hyper potions.

Sky pillar exists in Sapphire, Ruby and Emerald.

You may access it as part of the storyline but only after certain other events have occurred (the battle of Groudon and Kyogre in Sootopolis cutscene)

I believe you must go there to awaken Rayquaza to assist with defeating the other two pokemon.

If you fly to Pacifidlog town, surf east, following the rocks to your north. You will notice that a row of rocks that was blocking your way has moved. And changed, go north here and you should be at sky pillar.

You will need a Mach bike to get past the holes in all the floors (the second time you have to go there, I believe there arent any holes in the floors the first time you have to go there.) and take plenty of Max Repels as there are a lot of level 38-42 Pokemon in there (which will hinder your ability to get over the cracks in the floor, as Pokemon will keep attacking you when you are halfway through crossing and you will fall when the battle is over).

You won't get to battle Rayquaza the first time, I believe, but when you do go up to battle it, it is at level 70 and it is extremely fast! It is Dragon/Flying type. Take at least 30 ultra balls with you when you go, and try to put it to sleep or paralyse it. And definitely save before the battle so you can reset and try again if you fail an accidentally KO that pokemon.

To the east of pacifidlog, just surf and youll see it

Go to Pacifilog and then go east then north. NOTE: you can catch Rayquaza before you beat the Elite Four. I used him to help me through the 8th gym!