On the lab machines, I thought that if you tried to click the "Run" button, it would offer you a "Gradle..." choice.
However, on my Linux laptop at home, it's not doing that, so maybe your Mac is similar. Try this instead:
Right click on the whole starter-game-html project on the left panel, choose Run as -> Run Configurations.
Once that pops up, click on the green "Gradle Project" on the left side, and do "New Launch Configuration".
Name it something meaningful, like htmlLauncher and then enter html:superDev in the Gradle Tasks field. Try running it...
If it all builds successfully, then you should be able to open a web browser and put in localhost:8080/html/ to play the game in-browser. Any luck?