The ABC Import plugin is part of MuseScore main installation program since MuseScore 0.9.6. This plugin is an improved version compared to the one in MuseScore 1.2. You can now copy paste ABC notation directly in the plugin. It's still possible to load a file. The plugin should also deal better with bigger files.
The plugin requires Internet access since it calls the ABC2XML webservice to convert the submitted ABC code to MusicXML and then opens the MusicXML file in MuseScore.
MuseScore 3 has this plugin included.