Somehow I was unable to find this information on the web. Here is how to do it for those of you who don't like to use wizards.
You need following files:
Keep dll, pdb, and xml file in a one folder. Place Foo.addin file into "Visual Studio 2010\Addins" folder. Edit Foo.addin file to show a path to Foo.dll. Run Visual Studio. Plug-in should appear in an Add-in Manager. If you delete a Foo.dll file Visual Studio will rename a Foo.addin file to something different, so be prepared to rename is manually. And of course if a plug-in appears in an Add-in Manager, you can use it by selecting it.