Either way, you need to press button A to scroll thru your icons to Games (baseball bat pic) and then press B to select it.
You will see a Menu
Press A to scroll down to either PASSWORD (to enter your passwords) or SOUVENIRS (to view your souvenirs) and B to choose it.
If you've already entered the passwords, then use button A to scroll thru all the sourvenirs you have collected. The souvenirs are not for use just for looking at.
If you want to play with anything you have to use things from your ITEMS list ^_^
One more point to add to what TamaMum said: some items can be used only when your Tamagotchi character is an adult (for example only adults can use travel tickets), and some items can be used only when you've got another certain item (for example: you can't use the makeup unless you also have a mirror).