WordPress.org

I have successfully deactivated the plugin, but the visual tinyMCE editor doesn't show up!

You need to go to your Profile page, and check the "Use the visual editor when writing" option.

How can I add my own personal tagset to the plugin?

You basically need to create a new directory inside the /wp-content/plugins/wp_markitup/markitup/sets/ directory containing the toolbar stylesheet style.css, set.php which is the markItUp! toolbar specification and infos.php, a simple file used by the plugin to know if the tagset needs some external plugin to work.
Please note that set.php, despite its extension, it's a common Javascript source code file: the php it's just for localization purposes.

Your're strongly suggested to read the well written markItUp! documentation on how to setup your custom tagset, and watch a couple of markItUp! examples for yourself.

How can I add a new skin/edit an existing one?

Even easier. You just need to have some basic CSS knowledge and have to look at the wp_markitup/markitup/skins/ directory. You'll find a directory for each existing skin. Each dir will contain a stylesheet named style.css. It's common to see also an images directory used by the stylesheet itself.

I would like to become a WP MarkItUp! translator!

Great! You can start installing Poedit and opening the wp_markitup/languages/markitup-it_IT.po file with it. It's all pretty straightforward. If you have problems or you want to submit me the finished translation, don't hesitate to contact me.

Feature Request/Bug Reporting

If you find any problems/bugs/annoyances or if you feel like something really important is missing, than please let me know commenting at the WP MarkItUp! official page. Thanks for the support!