Did you update your game?
If so maybe try sparkIV, it's a little more user friendly. In SparkIV simply browse to the correct location (usually pc/models/cdimages then opening the right image) and hit import and select the files the mod provides. THen hit save and exit.
If you're keen on sticking with OpenIV though, just go to the right location.. go into edit mode (big button in top-right corner) and right click the files you have to replace and just select replace. Then select the corresponding files.