"Open" is never the correct action for any database file. Instead you need a front-end to a database engine, which can handle the file. If you are on Windows, such database engine might already been installed, if not, you can get it from Microsoft.
LibreOffice has some wizards to create such a connection. Try them out. Start Base and use the option "Connect to an existing database". In the drop-down list you find several options which are possible for Access, depending on the version of Access and the desired kind of connection.

Please use one of our support channel listed on http://www.libreoffice.org/get-help/community-support/ to get help. Find further useful general information about Base in "Base Handbook 4.0 (database)" from http://www.libreoffice.org/get-help/documentation/. To get access to the informations in your Access database file, you can try the guide in https://wiki.documentfoundation.org/Faq/Base/How_to_connect_to_a_Microsoft_Access_database, which is for older versions, but might still work for.