We’re talking about printing plugins that adds to the 4.4 printer handling system. To be clear: this should work on any 4.4.x phone.
It’s much easier than what it sounds like. It’s just to download the proper plugins from play store page for it. It’s btw the very same plugins for Nexus devices running 4.4, so it’s not a HTC idea of doing it this way and not something that HTC controls. So, if your printing system lacks a 4.4 printing plugin: contact that maker. But, then you might have an app that got you covered anyway.
Ok, back to the task at hand.
Simply go to Settings > Printing, 3 dot menu > Add service. Where you select and install the printing service you want to add. And then just Enable it, will most likely be a notification asking for this. That’s it!