If you have chrome installed just associated .swf to google chrome. If you are on windows installing flash player as mentioned is a option to

really cool stuff

Did you initiate the project with flixel-tools configured to IDEA? If so everything should be already set up properly.
If not run "haxelib run flixel-tools setup" and chose the IDEA option in the end so the next project initiate propaly. Use "haxelib run flixel-tools configure <project_directory> -ide idea " to set the IDEA projeto for you.