The D files should be treated as CGI programs. Add a CGI handler for the .d extension and enable the ExecCGI option. For example:

    <Directory /var/www/public_html>
        Options +ExecCGI
        AddHandler cgi-script .d
    </Directory>

Of course you need to change the directory path to match your setup. 

Also, be sure to check the permissions settings of the .d files. The server doesn't run the files as your user account, so they need to be executable by everyone. Running this in the folder with your D programs should do it:

    chmod a+x *.d