Install XFree86
 see
   http://xfree86.org and http://mrcla.com/XonX 
   on  installing  X Window System on your OS X

Download these:
ftp://gnu-darwin.sourceforge.net/pub/gnu-darwin/gtk+-1.2.8.tgz
ftp://gnu-darwin.sourceforge.net/pub/gnu-darwin/Mesa-3.4.tgz

Untar these with 
 tar xzvf gtk+-1.2.8.tgz
 tar xzvf Mesa-3.4.tgz

change to root , go into these directory and type:
make install
(This will install gtk and Mesa)

download this:
http://prdownloads.sourceforge.net/fink/dlcompat-20010123.tar.gz
tar xzvf dlcompat*
cd dlcompat*
make
make install  # do this as soot

cd GtkRadiant/libs/libxml2/
./configure
make
make install

cd GtkRadiant/libs/
make

cd GtrRadiant/tools/
make
(This will give you a q3map executable )

cd GtkRadiant/radiant/
make
(This will give you a radiant executable )