See, when you have created entity like Inventitemlocation, system should generate inventdim attributes like config, size, color field in staging table. those fields should mapped with your generateInventdimID method. your mapping and staging fields looks wrong to me. Please check DMFinventJournalEntity, you will get clear idea.
After that you can import the inventitemLocation with valid inventDim Combination, you should not enter inevntdimID value in file, your source should be size, color, InventLocationId etc. System will create the inventdimID automatically and mapped.
Hope it is clear now.