You need to ensure that row 1 of the csv you wish to import has the exact headings as required by Inventoria and that they are in the correct order i.e. "ItemCode, Description, Category, Location, Quantity, Cost, Sell".
When you import the csv the headings will appear on the left. On the right you will see the Inventoria headings and if you click to the right of one of these headings you should receive a lookup box with your headings. Just click the appropriate heading to create a match. Both left and right headings should now turn green. When all headings are matched click OK. Please note that any headings on the left which remain black are not matched and will not be imported.
I have also found (but it might just be me) that I can only import stock once. Any subsequent imports doesn't change the data. So you need to delete all existing data before you can import a new batch. Unless of course you have used "Bill of Materials" or any "orders" or "sales" in which case you cannot delete the stock items and therefore have to manually change those.