The recommended way to download and install Maps is with Composer using MediaWiki 1.22 built-in support for Composer. MediaWiki versions prior to 1.22 can use Composer via the Extension Installer extension.
Step 1
If you have MediaWiki 1.22 or later, go to the root directory of your MediaWiki installation, and go to step 2. You do not need to install any extensions to support composer.
For MediaWiki 1.21.x and earlier you need to install the Extension Installer extension.
Once you are done installing the Extension Installer, go to its directory so composer.phar is installed in the right place.