When uploading Customers, you get to the Match Your Data page. The instructions read, "Match the fields in your file to invoiceASAP. Then press Save." and there are two headings: Your File and invoiceASAP Fields. But there is nothing listed below this.
The file you are uploading probably has a space in the file name. This is what is causing the problem. The filename should be one word, without any spaces in it:
• WRONG: My Customer List.csv
• RIGHT: MyCustomerList.csv