Open IntelliJ and select "Open"
Navigate to the downloaded project and select the pom.xml. You may be prompted to open as a file or as a project - select project.
This is a typical view of IntelliJ with this project. You can use Alt+1 to open the project view you see on the left if it is hiding. Then you can expand the folders with the little cheese wedges
You need to rename the "username" package in entrants.ghosts and entrants.pacman to your username from the website. It should then look something like this.
You then need to edit the pom.xml so that the artifiactId tag matches your username.