Of what type you have declared the ‘filetext’ variable? If you just did ‘var filetext’ (instead of ‘var filetext : string’) then it’s dynamically typed, and tries to find methods at runtime.
WWW.data returns a string, and strings only have Split method with an uppercase ‘S’.
Code like this works:
var filedown = new WWW("http://www.unity3d.com");
var filetext = filedown.data; // implicitly types as string
var lines = filetext.Split( "\n"[0] );
for( var l in lines )
print( l );
The “\n”[0] takes first character of that string which is somewhat clunky but works.