Sorry, I would have explained it - but I have the german version. OK, I try it:
Only if you have activated the feature "keyfile" (which is also a bit hidden on the bottom of preferences where you change the Master-password), you'll get a new entry in the vault menu under preferences (3 points - see screenshot) - behind these 3 points you'll get the barcode.
I think it's difficult to find - I searched through the web and found nothing. Would be better, to find it in the tools-menu - or under vault-info, where the information is, if you have chosen the keyfile (f.e.). Sorry for my bad english - I hope you understand ;-)