First, beat the elite four. Then get at least 60 on your pokedex. Talk to oak, and he will give it to you.
The rainbow pass is a key item in Pokemon FireRed version. To trigger the events to get it, you first need to own 60 different Pokemon.
beat the elite 4
on island 1, in the pokecenter. u can get to island 1 by acheiving the rainbow pass
you have to find the ruby in mt ember and bring it to celio but first you need the national pokedex--Check this answer out: http://wiki.answers.com/Q/How_do_you_unlock_islands_after_beating_the_elite_four
Yes, Celio from One Island will give you the Rainbow Pass after you bring him the Ruby you find in Mount Ember. Of course, you can not get there unless you have beaten the Elite 4.
You have to go to a Pokemon event for the gba games (which they don't do since DS Pokemon came out) with a wireless adapter on your Gameboy SP or advance.
beat the elite four then talk to celio on island 1
You have to get the ruby and give it to celio. He will then give you a rainbow pass
It is not possible to obtain the rainbow pass in Pokémon Emerald. It is only available in Pokémon FireRed and LeafGreen.
by looking.
Find the Ruby inside Mt ember and then give it to Celio.
Talk to Celio on One Island. (I think you have to beat the Elite Four first)