I would do (after reading the mldonkey_init script too see what it does):
cp mldonkey_init /etc/rc.d/rc.mldonkey
then I make sure it is executable (if not chmod +x rc.mldonkey)
Last I edit the /etc/rc.d/rc.local file and add :
if [ -x /etc/rc.d/rc.mldonkey ]; then
So for any reason I wanted to not execute rc.mldonkey at boot, I simply chmod -x /etc/rc.d/rc.mldonkey, following slackware init script philosophy.