Opencart 2.0 Upgrade bug

Hi All,

I read somewhere that upgrading to the new 2.0 of Opencart was pretty easy......so took the plunge.
Followed the instructions and got a page of errors, when trying to log into Admin and the site, including :

'Notice: Use of undefined constant DIR_MODIFICATION - assumed 'DIR_MODIFICATION' in…'

I tried to do the config changes, as suggested, and now I just get blank pages for both the website and admin.

Am am ok with technical stuff, but this has given me a headache!

Any suggestions, now that I can't sell anything :-(

Regards,

Andy