This add-in doesn't show up on any tabs, it just adds a new export type. If you are using Office 2013, the installer for the add-in does not write the registry keys for that version of Office. This is easy to fix.
- Run the Registry Editor (regedit.exe)
- Navigate the following key: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office\14.0\Word\Text Converters\OOXML Converters\Export\MediaWiki
- export that key to a file
- Open the file with notepad and change the "14.0" to "15.0" and save the file
- Import the registry file
If you are running the 64 bit edition of Windows, do the same thing for HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Office\15.0\Word\Text Converters\OOXML Converters\Export\MediaWiki
Start Word and open or create a document. Select File-.Save As and when you get to the "Save As" dialog, you'll see "MediaWiki (*.txt)" in the list of supported formats in the "Save as type" list.