First, download the Lightroom Plugin here. Now unzip the download and place the plugin file in your Applications folder on Mac, or Programs folder on Windows. You can now use the Lightroom Plugin Manager to install the new plugin.
Once it's installed, you can double click the Plugin on the left side of your Lightroom window to set it up. Click Login or Sign up and enter the information requested. After clicking Save, Lightroom will sync your photos from 500px.