Sales invoice items are not under Customize. There is no separate tab for them. They appear only under Settings. But if you have them, they should be accessible under the Item field when you create an invoice.
When you click on the
Customers tab, do you see a list of your two customers? If not, you probably forgot to click Update when you created them.
If you go to
Sales Invoice Items, do you see a list with your item(s)? Again, if not, you probably did not click Update.
If you see both the customer and sales invoice item lists in the tests above, you should definitely see them in the dropdown list when you create a sales invoice. @lubos will probably have to look at your accounting file to see what is wrong.