Addon in one file
- Download needed addon/plugin script on local disk. If addon/plugin present as code, then copy all code and past it in text file, then save it with a .py extension, for example, "addon.py".
- In Blender open "User preferences" (shortcut Ctrl + Alt + U) and on "Add-ons" tab, click button "Install from file". Then navigate to the file you downloaded and select it.
- It should now appear in the window and you can tick the checkbox in the upper right to enable it.
If you would like to have the add-on enabled every time you start Blender, click button "Save user settings" at the bottom.
Alternatively, you can just save it and open it in the text editor and press Run Script.
Addon in several files
Also there have an alternative method that works well with multiple file addons as well:
Move or create an addon directory into the addons folder of your blender settings:
Example for Linux and Blender 2.69:
Example for Windows 7 and Blender 2.69:
Example for Windows XP and Blender 2.69:
C:\Documents and Settings\%username%\Application Data\Blender Foundation\Blender\2.69\scripts\addons
Example for Mac OSX and Blender 2.69:
Activate the addon in Blender.