Direct cash entries are associated with cash transactions regarding your company's registry (eg a loan, increase in capital etc.). Therefore, they aim to describe transactions that don't relate to sales or purchases documents. This is same reason why they don't participate in contact statements (they don't affect a contacts's balance) and the sales/purchases reports.
In case you received a payment that has to reduce a contact's balance, you might want to use an "advance payment" (no sales document issued yet) or the "new payment" function (provided there is a sales/purchase document that needs to be paid off) instead of the direct payment.
For further information on document-related payments, follow the instructions found in the related article. Also, if you wish to correlate an amount to multiple invoices, you can read the instructions about bulk document payments.