Hi Hamish, currently there aren't any tutorials for writing MoI scripts, since making tutorials requires quite a bit of work.

The closest thing you can do currently is to examine existing scripts to figure out how they work, there are a bunch collected here:
http://kyticka.webzdarma.cz/3d/moi/

and also every command in MoI has a script part of it that controls its logic flow (like when to show different buttons, when to pick points, when to select objects, etc...) and all those command scripts can be found in the \commands subfolder under MoI's main installation folder.

For getting up to speed on the Javascript language, probably a good focus for that would be to do some simple web pages with script logic in them, there should be quite a lot of tutorials available online about that, for example check out here:
http://www.w3schools.com/js/default.asp

But really it is very early on in MoI's lifetime for trying to use it from a scripting perspective, I just have not had enough time yet to focus time and energy on that area of MoI in particular so you'll be left more to yourself when exploring that type of stuff, although definitely check out Dave's stuff linked to above since he's gathered quite a bit of info there.

- Michael