In pclos /etc/init.d/ is a symbolic link to /etc/rc.d/init.d/ is where the scripts live, and /etc/rcx.d/ is a symbolic link to /etc/rc.d/rcx.d/ where the links for every script in the applicable runlevel live. The links for the scripts are generated by chkconfig based on the content of the script. If the script you have was not written for pclos it is not possible to install it reliably in pclos, because pclos has a second (and preferred) method for organising script execution on boot, to reduce boot time. Packagers adjust these scripts when packaging the application to ensure they will run as they should.
There are dynamic dns programs in the repo and you should install and set up one of those rather than trying to install one from outside the repo.