Shutdown and User button missing in panel

Ensure indicator-session is installed:

or with

sudo apt-get install indicator-session

And then log out and back in again.

Assuming you are on Xubuntu or using Xfce as desktop manager, there is no need to install indicator-session -- it will bring lots of extra packages to your desktop.

Instead add the Xfce plugin to the panel:

• right-click on the panel, then select Panel > Panel Preferences...
• select Items tab
• click on the [+] icon button
• look for "Action Buttons" and select it, then click Add button
• click Close button to finish

When you add "Action Buttons" to the panel, your username will immediately appear on the panel. Click on your username to see the actions (Lock Screen, Switch User, Suspend, Shut Down, Log Out...), which are available in the drop-down menu.

Works for me on Xubuntu 15.10 (fresh install).