In the screenshot from the KDE website, the top line says “File Project Device Tools Settings Help”. When I open K3b on my Acer laptop running Mint 19.1, that line doesn’t appear. (Sorry, I don’t know what that line is called) I assume there is a way to toggle it on and off, but I can’t find it. Normally, that’s where I would start. There must be an instruction page somewhere.
CTRL+M should toggle the menu bar
When I click the Help button and select K3b Handbook, I get:
The URI ‘help:k3b/index’ does not point to a valid page
K3B is disk burning software, right? I used this years ago when I still burned stuff onto CD/DVD.
By default it shows up with the menu, Ctrl_M toggles that on and off…
However, unlike you, if I press F1 I do see the handbook… perhaps because I’m using Plasma desktop.
So your distribution likely only installed the minimum k3b installation, not the full Plasma environment 
so you can choose to either install the whole khelpcenter or you can just try a search online: