I installed OPC on v1.5.8 test site yesterday. On checkout the following headers are not shown properly. I can't seem to figure out how to insert the defines for these items.
MODULE_ORDER_TOTAL_COUPON_TITLE
MODULE_ORDER_TOTAL_GV_TITLE
Next to the input box the header there is missing these defines
MODULE_ORDER_TOTAL_COUPON_TEXT_ENTER_CODE
MODULE_ORDER_TOTAL_GV_TEXT_ENTER_CODE
MODULE_ORDER_TOTAL_GV_USER_PROMPT

I searched these in the forum but they didn't return any results.