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Do I have to add any code in the template?

No, the plugin will automatically add the meta tag in the header.

How I can be sure that the plugin works properly?

Go to the frontend of your page and check the source code.

Will this plugin works together with other SEO plugins?

Yes, for sure. But check your code for double entries.

What to do when the plugin is not shown on the edit page?

Check if 'Meta Robots'are visible under 'Screen Options'.