First download the virtual box, download it for Mac and install it. Now download the Chrome OS and extract it. Now launch your virtual box and click new. A "create new virtual machine" wizard will open, click next, put the name of the OS and choose Linux in operating system and Ubuntu in version now click next. Now give memory atleast 1000 MB and click next. Select "use existing hard disk" and tick the "boot hard disk" and choose the OS file by clicking the folder icon next to it, click add and locate your OS file, click next and then finish. Now click on start and OK. After that, enter your Gmail username and password and press the enter key and you are done.