If you are using the ModdingWay patches, the classic kits are often added directly to the team selection screen or the creation centre.
For an offline, "no-mod" solution, you can force the game to recognize all unlockables as active by modifying the game's internal initialization files. how to unlock classic kits in fifa 14 pc offline
Classic kits are stored as assignable items. Look for entries where the assetId corresponds to classic kits (usually IDs between 140000 and 145000). To make this easier: If you are using the ModdingWay patches, the