This Post will help you on how to enable the Alert Me button in SharePoint Library or List ribbon.
So, you have done with fresh SharePoint installation and create Web App, root site collection. Its time you start exploring Document library options.
You think of configuring a alert but you noticed the "Alert Me" option missing from your ribbon.
To enable the button, Go to Central Admin > System Settings > Configure Outgoing email settings.
Enter the details of SMTP server and addresses.Click OK
Happy SharePointing :)