x11

I have a black macbook with the newest version of OSX (Tiger not Leopard) I've installed all updates. I'm trying to download Gimp. It says i need x11. I went to install x11 from the pkg and it says I can't because i have newer software already on my computer. The Gimp website said Gimp should run automatically if I had x11 on my computer. When trying to install Gimp it tells me to find x11 and it has a browser pop-up showing my applications. I have looked everywhere to see what I should do. Please someone take me on a step by step laymen's terms version of how to install Gimp.