In the "stable" (i mean the version that is available on the Blender.org download page, it does not mean it's more stable than the development version actually) version of Blender, you have already a lot of addons.
They are available by clicking on File -> User Preferences and go to the Addon panel.
From there you can select an addon and enable it .
Now in the "stable" version of Blender there's a whole category of addon that is removed, the "Testing" category that is empty if you click on it (only the "Official" and "Community" categories have addons available on "stable" version).
That's because the Testing category is for contrib addon that may or may not work and the devs do not have time to check every single of them.
Now on development version of Blender all those contrib addons are part of the download , so by example if you download an up to date with the current development version of Blender , in the Addon panel of those if you click on the Testing category you'll have access with a very lot more addons :
buildbots version of Blender can be downloaded there :
http://builder.blender.org/download/Now official or contrib addons do not mean every single addons existing.
As even the contrib addons are listed only the addons their developer have uploaded on the blender foundation servers.
There's a lot that aren't.
You should find most of them through the addon board on Blenderartist.org :
http://blenderartists.org/forum/forumdi ... and-ThemesNow when you download an addon it is in .zip format.
If you know what you're doing you can install the addon manually, by extracting it and moving the file to
...\2.69\scripts\addons\
or
...\2.69\scripts\addons_contrib\
But as you're new to Blender, i suggest that you use the other method :
Go to File -> User Preferences -> Addons
On the bottom of the addon panel, notice the "Install From File" button, click on it and point the blender file browser to the .zip file of the addon you downloaded .
Note : when you enable addons, if you don't click on the bottom of the addon panel on "Save User Settings" , the next time you'll launch Blender those addons will be back at NOT being enabled.
It's good when you're experimenting and do not really know which addons you need, but when you have a real need of a specific addon at every of your Blender usage, it's better to have it being automatically enabled at launch, so you don't have to do it manually everytime