This Widget is provided by the Jetpack Plugin.
To add it:
- Activate the Jetpack Plugin
- Activate Extra Sidebar Widgets in the Jetpack->settings section of the Dashboard.
- Add the widget on the Widgets section of the Dashboard Appearance->Widgets.
- You can add your twitter name to show a profile or an id to show a search. NB as of September 2018 twitter have changed their service again, you can no longer use the ‘Widget ID’ option.
More details on the jetpack blog: