how do I charge an open invoice to loss

If you mean you want to write them off as the invoice because it won't be paid, you can do this with the Receive Payments feature, by discounting the invoice 100% using your write off account, like this:

However, this will write the invoice off as of the invoice date (you'll see the expense on your Financial Reports on that date.)

If you want it to be on a different date, create a Credit Memo for the customer, and use an item set up for this purpose (probably an other charge type item) that uses your write off account. As you record the Credit Memo, you should be prompted and asked what to do with it. Choose the option to apply it an Invoice. Then the credit should appear on your Financial Reports as of the Credit Memo date.