Ok, I recently started playing my FireRed again because I'm trying to get all the Pokemon in my Diamond Pokedex and I need Suicune, Entei, and Raikou. I beat the Elite Four, gave Celio the Ruby, and have access to all seven islands (not including Birth and Navel.) My problems are that when I go to the Waterfall Cave or whatever on island 4, I can't get the Rockets and Lorelei to show up even though I went up the waterfall, and when I get to the Ruin place on island 6, the scientist dude is in front and keeps saying "hey don't be jealous I discovered this place bla bla bla..." and I can't use cut to get in. I've looked at several walkthroughs online (including Psypoke), and my stupid Prima guidebook doesn't show islands 4-7. Please, please help me if you can because this is really annoying and I need to get Suicune!!

I'm guessing there's some NPC you need to talk to to activate the next part of the quest, maybe try Celio again.

Nevermind I managed to figure out the Team Rocket thing and gave the Sapphire to Celio. Is that all I need to do to get Suicune to be in the wild though cause I've looked for hours now, and no sign of him.

Finding Suicune is a matter of luck, but yes, after you can trade with RSE, you should be able to find it.