Hello, this is how to play minecraft on a usb stick:
If you want to move all the saves:
1.On the main computer locate your minecraft folder by opening finder, clicking "Go", then HOLDNG alt/option, clicking Library, then app support, then minecraft
2.Drag "saves" to the usb stick.
On all target computers:
1.Download and run the minecraft launcher.
2.Tick "Game Directory" and type "/Volumes/usb-name-here" with usb-name-here as the usb sticks name.
Note that if you want to put other files on the usb stick, at step 3 you should add "/minecraft" to the end of "Game Directory" to put it in it's own folder.