First find the Safari.app icon, then right click on it and select "Show Package Contents" from the contextual menu; in the window that opens double click on the Contents folder, then Resources, then find the folder labeled English.lproj; the easiest way to do this is to make sure the window has focus and type in "E".

Now when you have found the folder double click it and find the file "Browser.nib" this is the one that you will be replacing with the .nib you Dled from me; make a copy of that file and either move it somewhere safe, or just rename it Browser-old.nib, or BrowserMetal.nib... you get the idea; then drag and drop the new Browser.nib into the English.lproj folder and click yes when the replace dialog comes up.

That is all there is to it. Now you have the much nicer Aqua interface, and still have the original .nib if you ever want to go back. I will be creating a .PDF of this tut complete with pictures for DL later, so stay tuned if this is still confusing.