Until that particular plugin's creator gets off his duff and releases a new version of the plugin that will automatically make the icon appear on the toolbar ...
You can place custom icons in the plugin's preference folder to override the icon that comes with the plugin (or to give an icon to one without one).
48x48 pixels named "plugin.png".
Edit->Preferences, click the "Open Preferences Location" button at the bottom-right of the dialog, then open the plugin_prefs folder and look for the plugin in question.
You can also put plugin.png in the main plugin folder, but this can get overwritten anytime you remove/update the plugin.