Agreed, I spent a few minutes looking around and trying to add nodes before I found the help. And then some time looking for an 'insert' key on my keyboard... haven't found it yet ;)
Wow, I never thought that an Insert key is going to be such mystery. It is kind of a standard way of inserting a new node in other mind mapping (desktop) apps...
If it makes you feel any better I have used other mind mapping tools and Insert was intuitive to me.
I also have no "insert" key on my keyboard. And tab just cycles through various form controls on the right..
fn+return does the trick on macs.
Tab works once you focus the map itself by e.g. clicking a node.
And besides not being included on every keyboard, it's not an command key, but a text-mode toggle, in every other desktop app.
I guessed (correctly) that on my Macbook Air's keyboard cmd+return is "insert".
Unfortunately cmd+return does not work on my iMac (it does the same thing as return, which is add sibling).