Right, I've had that error message myself. Here are some things to check:
1. I suppose you used the item editor to add the mesh to the helm. So, open item editor and find your helm, now check the spelling in the field "mesh name". Make sure that you spelled the mesh name in exactly the same way as your .brf file. It takes into account lower and uppercase letters.
2. Open up your module.ini in your modules folder. Search for a line where it says load_mod_resource = name_of_your_mesh. If you can't find that line, add it in the bottom of the file.
3. Open the item_kinds1.txt file and scroll all the way down. Look for the name of your mesh (it should be in the last few lines, provided that you haven't added any other items to the game earlier). If you can't find your mesh somewhere, you must have done something wrong when you were fiddling with the item editor.
Hope that helps!