Did you set the respective parameter to true? The parameter is called "Add button to main menu".
I tried to reproduce the issue, but still, I can't. When I put the plugin in a new project and don't change any parameters, the button appears. Open the console (via F8 or F12, switch to console-tab) and check it for errors. You probably set a parameter to an uncorrect value. That can cause an error and the plugin doesn't work at all.