I am helping with the books for a small diner and most items are purchased with cash or COD. How can I track these expenses? How do I enter the receipt/invoice? QB won't lets us open a cash account. Many purchases are made at K-Marts and Wal-Marts for cash. Any assistance would be appreciated.

Yes, you can have a 'Cash' account in QB. Just set it up in the Chart of Accounts as a 'Bank'-type of account. When you record your receipts, any cash you take in that is not deposited to your actual bank/checking account would be deposited to this Cash account. Then when you pay out cash for supplies, COD's, etc., just write a check from this Cash account (put COD, Payout, or a similar notation where the check number goes). Post it to the appropriate expense account (supplies, postage, delivery charges, etc.)