Modern Merchant can be checked out from its Subversion repository. Here is the svn command:
svn co https://modern.svn.sourceforge.net/svnroot/modern/trunk/htdocs modern

(Enter the above all on one line)

This creates a directory called modern that is very similar to an unpacked release package. Copy all files (include .htaccess) to your web documents root. With a web browser, visit /mminstall.php on your site. Follow the installation wizard.

More information on the SVN repository can be found at the SourceForge Subversion page for Modern Merchant

Click here for the browser-based SVN browser.