After reading this: https://help.ubuntu.com/community/Applets
it says to enable an applet right click on an empty part of a panel and choose "add to panel".
I am unable to find anywhere that I can right click to get that option. There is a bar across the top of my screen, I assume that's the panel to right click?
I'm running Ubuntu Budgie.
The guide you mentioned is written for GNOME 2 desktop environment for an old Ubuntu release (Ubuntu 7.04). So it may not be the right guide to follow to add applets to Budgie desktop in 2018.
To add applets to Budgie, go to the top-panel and click the square icon at the very end of the panel to reveal the settings pane. From here, click the gear icon. The gear icon opens to the general settings area for the desktop. Then go down to the “applets” section of the settings, and click the + sign. This will open a menu, where the user can select a new applet to add.
To remove an applet, click on the desired applet, then click the – sign to remove it.
It is also possible to re-position the applets. Highlight an applet, then click the up and down arrows to move it around.
(Source)