Meat tenderizer refers to a powdered naturally derived enzyme powder. The enzyme most commonly used is papain, which comes from papayas or bromelain, which comes from pineapples (a tropical fruit in the bromeliad family). Meat is sprinkled with the powder, and the enzymes help to break down the meat fibers. The same result can be obtained by marinating meat in papaya or pineapple juice.
Tenderizer products can be purchased with or without seasonings.
Sometimes the term meat tenderizer is used to describe a meat mallet, which is alternately used to pound and flatten meat to make it more tender. These are readily available in any kitchen shop or even at your grocery or hardware store. You also just use the side of a small plate or saucer, which works pretty well too.
Purchase powdered meat tenderizer in most mainstream grocery stores. A very well-known, reliable brand is Adolf's. You can also find it online at Amazon: Adolph's Meat Tenderizer.
If you don't have meat tenderizer or just don't want to use it, you can use one of these substitutes.
salt, dextrose, bromelain (tenderizer), and calcium silicate (added to make free flowing).
Dry tenderizer products are typically added to the meat just before cooking. The meat should be moistened first with a little water before the product is sprinkled on. You only need a little, about 1 teaspoon per pound of meat. The products contain salt so you should not add more when cooking.
The meat is then pierced with a fork about every 1/2" to allow the product to penetrate the meat. If you're tenderizing a thick cut of meat then allow the meat to stand for at least 30 minutes before you cook it.