I assume you've already placed the required resource and texture files within their proper folders and that you've added relevant lines to the module.ini and item_kinds1 files.
What you may have missed is that the item_kinds1.txt has a count at the top of the file. Make sure you increase the item count for each item you add. Also, bear in mind that position within item_kinds1 matters. If you add new items this way, add them at the end. Unless, of course, you're replacing existing items.
If you're trying to add something with heraldry that's going to require more work. You'll need the sourcecode (available for free) and a version of Python that works with it (2.6.5 worked for me). You'll then have to add proper lines to the module_items, module_tableau_materials and module_meshes python files and compile them to get heraldry to function properly.
