No need to download the sapling addon, it's already part of blender. Press Ctrl Alt U, go to the Plugins tab, type sapling and enable it.
If for any reason you need to download the addon the link is here: http://wiki.blender.org/index.php/Extensions:2.6/Py/Scripts/Curve/Sapling_Tree
For other plugin downloads refer to:
http://wiki.blender.org/index.php/Extensions:2.6/Py/Scripts
and
http://blender.stackexchange.com/a/15356/1853https://blender.stackexchange.com/a/15356/1853
One last thing to watch out for is that when you download puffins they are in a text format as .py scripts and not as HTML