When I try to add the PPA, I get this error:

Code: Select all

 $ sudo add-apt-repository ppa:kdenlive/kdenlive-stable
'This PPA does not support trusty'
Cannot add PPA: ''This PPA does not support trusty''.


I'm running Mint, 17.3 I believe. Is there another PPA I need to use for this, or have I messed up somewhere else?

(disclaimer: I am a Linux noob, so it is very possible that the solution is pretty simple. I just don't know it yet.)
{maybe that should be my forum signature ....}

Your distro is too old.
For 2 years Kdenlive needs Qt5 & KDE Frameworks 5 that have been introduced in vivid (15.04), recommended now is at least xenial (16.04).
You can download AppImage that should work in your environment.