In the below file can find the code:-

addons\web\static\src\xml\base.xml

Line no:480

In the UserMenu template uncomment the below code:-

Example,

        <!--<li><a href="#" data-menu="about">About OpenERP</a></li>-->

EDIT:-

Additional code related to About Openerp:-

addons\web\static\src\js\chrome.js

Line no 1149:-

on_menu_about: function() {
        var self = this;
        self.rpc("/web/webclient/version_info", {}).done(function(res)    

});

addons\web\controllers\main.py

Line no: 679

   @openerpweb.jsonrequest
    def version_info(self, req):
        return openerp.service.web_services.RPC_VERSION_1