You can ask on my Discord, ask me on Mastodon, or open a discussion on GitHub.
If you run into something that you think might be a bug then you can open an issue (make sure to include your Godot version and Dialogue Manager version).
There are a few ways you can support the development of Dialogue Manager. You can become a patron on Patreon, buy me a coffee on Ko-fi, or become a GitHub sponsor.
If you're not in a position to do any of those things then you can just give me a sub or like on YouTube.
One of the most common causes is that you've implemented player movement inside _process instead of in _unhandled_input.
For more of a guide then check out the code for my beginner dialogue example project and the video that goes with it.
There is a Project > Tools menu option to create a copy of the example balloon into somewhere in your project (never edit the original example balloon directly because any changes you make will be overwritten when updating the addon).
From there it becomes an exercise in UI building using mostly Godot's UI control nodes (with the exception of the provided DialogueLabel node). I recommend digging through the initial code to familiarise yourself with how it works before changing anything.
To comply with the license you just need to include the license text (or a link to it) somewhere in your game, usually at the end of the credits.
If you want to also credit it specifically then you can include something like "Dialogue System by Nathan Hoad" or whatever (I'm not fussy).
The short answer is that not all games need to have any kind of dialogue, let alone branching dialogue trees so it would just be introducing bloat into the engine for little benefit. Another good reason to have it as an addon means I can iterate on it much faster than having to wait for engine releases.