I have just added the facebook comments plugin and I am seeing a first comment of a user.
But if I go to http://developers.facebook.com/tools/comments there isn't any recent comments.
Any clue?
You will want to make sure you've specified your Facebook App ID in the header section:
<meta property="fb:app_id" content="{YOUR_APP_ID}">
This will give administrators of your application in-line access (in my experience it takes a bit to show up, possibly due to caching).
Keep in mind that Facebook recommends this approach over doing an admins meta when using multiple boxes. From Facebook seen here, scroll to Moderation Tools:
If your site has many comments boxes, we strongly recommend you specify a Facebook app id as the administrator (all administrators of the app will be able to moderate comments). Doing this enables a moderator interface on Facebook where comments from all plugins administered by your app id can be easily moderated together.
You can access this comment tool by going to the following link, after inserting your APP ID:
https://developers.facebook.com/tools/comments?id={YOUR_APP_ID}&view=queue
Or by viewing the index here.