Reinstall MacOS on a Macbook that doesn't have MacOS anymore?
You can obtain the latest Build of El Capitan (10.11.x) directly from Apple via this link.



As for creating a bootable USB installer, I would recommend converting the dmg to an iso via AcetoneISO.

It'll convert the DMG to an ISO for you. After that, create the bootable USB  with your new yourmacos.iso using DD.

dd if=/path/to/yourmacos.iso of=/dev/sdX &amp;&amp; sync


I don't know if you are familiar with this but dd does byte by byte copying and 
sync ensures that all data has been written to your USB (so you can savely unplug/eject it afterwards).