'Allo deary. So ye say it cannot be found. Can ye locate the supposed missing file in y'er materials folder f'er me? If this was from a downloaded mesh pack, it could be very possible that 'twas f'ergotten and not placed inside the folder when the download was created.
Also, if FeralHeart cannot find the file, ye are most likely not able to see the material in-game y'erself. So what is probably occurring is when FeralHeart goes to export the object group this file is included in, it is not there and the error pops up. F'er example if the file is in MyRocks and ye 'ave used rocks in y'er map, when the game goes to export the rocks group, it notices that a file is missing in the group and warns ye.
I 'ave a few object groups that give me this error every time I export a map. I know the mesh/material isn't there, so I just ignore it. I wouldn't be too concerned. If the file is missing, ye can't use it anyway. Therefore, 'tis like 'tisn't there in the first place.
But if the missing file is located in the correct folder, ye may 'ave a problem on y'er hands. Hope this helped ye, deary!