If the item code field not showing while generating invoice, you can try the following steps:
Step 1:Make sure,the item code feature should be enabled in your BUSY.If its not enabled,then
Step 2:Go to Administartion menu,
Step 3:Click on configuration,
Step 4:Click Features and Options,
Step 5:Click Inventory tab,
Step 6:Enable Party wise item-code,and save this window.
Step 7:Then go to the Administration and click configuration,
Step 8:Here you can find an option Party wise item-code,click on it,
Step 9:Then select the party and specify code for different items.
Step 2:While generating a voucher Place cursor on an item and press F4 then window will appear, click Party item-code,here you can pic the party item code.