You can toggle this when you go into 'install zip from SD card'. If you select 'toggle signature verification' it will display 'enabled' and then select again and it will be 'disabled'. That should work. If it doesn't, don't flash that ROM, try downloading it again, check MD5 sums, check that it is a ROM for your phone, and if all are the same, have another go.
Sent from my ST18i