I am building asterisk freepbx server and trying to install dahdi-linux-complete-2.1.0.4+2.1.0.2 and when try to run "make" command, i am getting :

make -C linux all
make[1]: Entering directory `/usr/src/dahdi-linux-complete-2.1.0.4+2.1.0.2/linux'
echo "You do not appear to have the sources for the 2.6.18-8.el5 kernel installed."
You do not appear to have the sources for the 2.6.18-8.el5 kernel installed.
exit 1
make[1]: *** [modules] Error 1
make[1]: Leaving directory `/usr/src/dahdi-linux-complete-2.1.0.4+2.1.0.2/linux'
make: *** [all] Error 2

i check my linux kernel with uname -r command:
2.6.18-8.el5

The kernel sources are located under /usr/src directory, they are usually installed as part of the development packages for you distro, you may not have installed them during the installation, most distros have an update gui that will allow you to install or remove software.