Magnolia 5.3 reached end of life on June 30, 2017. This branch is no longer supported, see End-of-life policy.
The RSS Aggregator module is bundled with Magnolia and typically already installed. You can download it from our Nexus repository
RSS Aggregator is a Community Edition module. It is typically already installed. Launch the Configuration app and go to /modules/rssaggregator
to check.