Before you can log out of Dropbox on your PC, you'll need to open the application. To do this, click on the Dropbox icon found in your taskbar or start menu. Once the app is open, click on the icon with your profile photo in the upper right-hand corner:
Just like on PC, the easiest way to log out of Dropbox on Mac is to first open the app. Click on the Dropbox icon in your taskbar, then select the icon with your profile picture in the upper right-hand corner.