Qnewbie

RB450G: (screw hard!)
http://www.jhpeng.com/2012/11/routeros-rb450g-reset.html

RB750G:
http://forum.mikrotik.com/viewtopic.php?f=3&t=34724

But when I disconnected power cord and Hold reset button until I connected power cord in to the DC jack, then the "ACT" LED Blinks and finally my router OS reset as well as Default factory.

or in more clear text:
http://klseet.com/index.php/mikr ... set-factory-default