You want the "links bar", which is situated at the right hand end of the tab bar...
You can either right click there, and choose "Manage Links", or from the menu, go "Tools > Links > Manage Links"
Now, without getting into heavily complex explanations, hopefully a few examples will help get you up and running. The beauty of these things is that they only show up if the files match the string provided, so, for example, video related links won't show up for a list of audio files. All of these examples open in an external browser, but with a click of the mouse, you can set them to open within MC if you prefer that.
To get these examples onto your links bar, copy all of the text in the code box, open the links manager as detailed above, click the "Add" button, then click on the "Paste link information from clipboard" button...
IMDb - If a file is tagged with the IMDb ID, this will open the page directly, if the IMDb ID is missing, it does an IMDb search for the title:
<Link version="1.2">
<Item Name="Name">IMDb</Item>
<Item Name="Action">1</Item>
<Item Name="Base URL">http://www.imdb.com</Item>
<Item Name="Search URL">if(isempty([IMDb ID]),http:////www.imdb.com//find?s=tt&q=Hexify([Search]),http:////www.imdb.com//title//[imdb id]//)</Item>
<Item Name="Filter">[media sub type]=[Movie],[TV Show]</Item>
</Link>
Rotten Tomatoes
<Link version="1.2">
<Item Name="Name">Rotten Tomatoes</Item>
<Item Name="Action">1</Item>
<Item Name="Base URL">http://www.rottentomatoes.com</Item>
<Item Name="Search URL">http:////www.rottentomatoes.com//search//full_search.php?search=Hexify([Search])</Item>
<Item Name="Filter">[Media Sub Type]=[Movie]</Item>
</Link>
Music Video - Searches YouTube for the music video for the selected audio track:
<Link version="1.2">
<Item Name="Name">Music Video</Item>
<Item Name="Action">1</Item>
<Item Name="Base URL">http://www.youtube.com</Item>
<Item Name="Search URL">http:////www.youtube.com//results?search_type=&search_query=Hexify([Artist]+[Name])</Item>
<Item Name="Filter">[Media Type]=[Audio]</Item>
</Link>
Artist On AMG - Searches all music.com for the selected artist:
<Link version="1.2">
<Item Name="Name">Artist on AMG</Item>
<Item Name="Action">1</Item>
<Item Name="Base URL">http://www.allmusic.com</Item>
<Item Name="Search URL">http:////www.allmusic.com//search//artist//Replace([artist],/ ,+)</Item>
<Item Name="Filter">[media type]=audio</Item>
</Link>
Album On AMG - Searches all music.com for the selected album:
<Link version="1.2">
<Item Name="Name">Album on AMG</Item>
<Item Name="Action">1</Item>
<Item Name="Base URL">http://www.allmusic.com</Item>
<Item Name="Search URL">http:////www.allmusic.com//search//album//Replace([album],/ ,+)</Item>
<Item Name="Filter">[media type]=audio</Item>
</Link>
Track On AMG - Searches all music.com for the selected track name:
<Link version="1.2">
<Item Name="Name">Track on AMG</Item>
<Item Name="Action">1</Item>
<Item Name="Base URL">http://www.allmusic.com</Item>
<Item Name="Search URL">http:////www.allmusic.com//search//song//Replace([name],/ ,+)</Item>
<Item Name="Filter">[media type]=audio</Item>
</Link>
See if that's enough to get you rolling... any questions, fire away...
-marko