How to find and download a missing plugin?

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http://askubuntu.com – I tried to add a radio station to the radio player of Rhythmbox, so I clicked "add" and copied the URL but it won't play when I click on "play". A window comes in the first place searching for a plug-in then it tells me (something like) that: python 2.7 is missing a plug-in type text html decoder When I open the properties of the radio added it tells: Your Gstreamer installation is missing a plug-in. How can I fix this ? (HowTos)