Your tamagotchi needs to be an adult to wear the make-up.
You also need a mirror in order for the make-up to work properly. You can buy one in the shop. The mall in TamaTown doesn't have a mirror in it, so you'll just have to wait for the shopkeeper to start selling it. You can change the shop's items by setting the time to 2:59pm or 6:59pm and waiting one minute.
Once you get a mirror and your tama is an adult it is finally ready to use the make-up. Select the make-up from your items list and the mirror will pop up with your tama looking in it. After a while the mirror and your tama will disappear. Then your tama will come up close to the screen to show you the make-up on him/her. Sometimes, though, tamagotchis will be shy and they'll only show you their eyebrows.
The make-up will look different on each character.
The make-up is not reusable. If you want to see your tama with the make-up again you will have to buy it again. Wait for the shopkeeper to sell it or you can go to TamaTown's mall to buy it. It's next to Kuchipatchi on that little table in the Forever Tamagotchi store located on the far left of the mall.
GotchiGirl96