It's pretty simple - first, make sure the USB registers with the computer (shows an icon on the desktop). After that's complete, look for the Minecraft application itself - should be in your application folder. Drag that to the USB. Subsequently, go to spotlight (the little magnifying glass in the top right corner of the screen) and search for 'Minecraft.' This should bring up the folder of saves, textures, and the like. Click 'show in finder,' and viola, your Minecraft folder is on the screen in front of you. Now drag that bad boy to the USB. Finito!
In summary: find the Minecraft application, drag to USB. Find the Minecraft saves/.jar/textures folder, drag to USB. Done!