Over the years a number of people have reported getting a 503 error message when trying to install their newly-downloaded MODX.
Go to the setup directory in your browser - yourdomain.com/setup. This will run the /setup/index.php file, and start the installation process.
What's the problem?
Before it is installed, MODX cannot find its configuration file, which is not surprising as it doesn't yet exist. The installation process will create the file and write the necessary configuration to it.
In some cases, MODX may have a problem creating this file. In cases like that, you may need to actually create an empty config.inc.php file in the core/config/ directory, making sure that it's writable by MODX (PHP), then run the /setup/index.php installer.