I've seen the same issue in more than one non-Explorer file manager as well, so it's not unique to MultiCommander. In most cases, an alternate method to pin any program to the taskbar is to simply drag the shortcut or executable to the taskbar and drop it between the buttons where you want it. Just don't inadvertently drop it on top of another button or the effect may be quite different. If you drag the executable file, it automatically creates a shortcut for the taskbar and you can modify the properties after it's placed there.