root@sashok-virtual-machine:/home/sashok# ln -s /opt/dklab_realplexor/dklab_realplexor.init /etc/init.d/dklab_realplexor
root@sashok-virtual-machine:/home/sashok# update-rc.d dklab_realplexor defaults
update-rc.d: warning: dklab_realplexor stop runlevel arguments (0 1 6) do not match LSB Default-Stop values (1)
 Adding system startup for /etc/init.d/dklab_realplexor ...
   /etc/rc0.d/K20dklab_realplexor -> ../init.d/dklab_realplexor
   /etc/rc1.d/K20dklab_realplexor -> ../init.d/dklab_realplexor
   /etc/rc6.d/K20dklab_realplexor -> ../init.d/dklab_realplexor
   /etc/rc2.d/S20dklab_realplexor -> ../init.d/dklab_realplexor
   /etc/rc3.d/S20dklab_realplexor -> ../init.d/dklab_realplexor
   /etc/rc4.d/S20dklab_realplexor -> ../init.d/dklab_realplexor
   /etc/rc5.d/S20dklab_realplexor -> ../init.d/dklab_realplexor
root@sashok-virtual-machine:/home/sashok# update-rc.d dklab_realplexor start
update-rc.d: warning: dklab_realplexor start runlevel arguments (none) do not match LSB Default-Start values (2 3 4 5)
update-rc.d: warning: dklab_realplexor stop runlevel arguments (none) do not match LSB Default-Stop values (1)
Use of uninitialized value $argv[1] in pattern match (m//) at /usr/sbin/update-rc.d line 297.
update-rc.d: error: expected NN after start
usage: update-rc.d [-n] [-f] <basename> remove
       update-rc.d [-n] <basename> defaults [NN | SS KK]
       update-rc.d [-n] <basename> start|stop NN runlvl [runlvl] [...] .
       update-rc.d [-n] <basename> disable|enable [S|2|3|4|5]
        -n: not really
        -f: force

The disable|enable API is not stable and might change in the future.