How can I use it? I can't find any menu or icons or even $ or [ ] (described in mathJax docs) doesn't work.
New feature! Inline Math! But..
@appleparan it's available in the most recent beta version, yes. We'll add a menu item and toolbar item for it still in another beta, as well as documentation here on the support site. In the first beta we focused on it being importable from TeX.
CEO & Co-founder of Manuscripts.app Ltd