If you are using Gatekeeper on your Mac (it’s enabled by default), it may complain because you are trying to run an executable downloaded from the internet.
To launch the game (after installing the Playdate SDK), you will need to right click the file “Poker Poker Magic.pdx”, select “Show Package Contents” and then right click on “pdex.dylib” and select “Open”. The warning popup will show again but this time it will have an “Open” option. Select it. Now you will be able to run the game by double clicking on “Poker Poker Magic.pdx”.