Hi Dmitry, there are some instructions on how to install plug-in commands here:
http://moi3d.com/faq#Q:_How_do_I_install_a_plug-in_script.3F

> I tried to install some plugins from the page of Max Smirnov (I extracted them to the
> 'commands' folder of Moi) but as far as I see, nothing new appears in the ui of Moi.
> Sure, something I do is wrong but what?

Currently that's normal that installing a plug-in does not make anything appear in the UI, you need to set up a shortcut key to trigger the new command. Or also as mentioned above you can press the tab key and type the command name into the XYZ input box and push Enter to launch it.

But if you install Max's CustomUI extension, that does have a scripts menu in it that will list new commands that you add.

- Michael