If you have installed it via the itch.io desktop app then you can simply use the 'Launch' button and click the 'mannequin' option.
If you downloaded the ZIP file, then you can run 'mannequin' file as an executable via the command line:
Note: as an alternative to this GUI way, you can also type 'chmod +x mannequin' before running it using './mannequin' command
Hope this helps, and feel free to let us know if there's still a problem. Thank you very much!