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 on the command line in the folder with your D programs should do it:

    chmod a+x *.d

Because the files are now executable, they will be run using the program listed in the shebang line of each script. This is where rdmd comes in. The first line in each file should be something similar to this:

    #!/usr/bin/rdmd

I don't know the specifics for the D language. Be sure and use the path to your rdmd compiler. If your not sure run this on the command line:

    which rdmd

That should tell you where it is.