I have a webpage on which I added a facebook comments plugin.
<fb:comments href="http://mywebsite.com" num_posts="20" width="510"></fb:comments>
How would I be able to moderate those comments? "mywebsite.com" is not actually the address where the page with the comments plugin is available.
You also need to include the fb:admins and fb:app_id tag in the header of the site that is using the comments plugin:
<head>
...
<meta property="fb:app_id" content="APP_ID"/>
<meta property="fb:admins" content="USER_ID"/>
...
</head>
Once you've done this you will see the moderate links on the comments box. You can also navigate to https://developers.facebook.com/tools/comments to see a list of all your apps that have comments plugins enabled.
More information here: https://developers.facebook.com/docs/reference/plugins/comments/