After writing the actual code you also need to click Edit script details to make sure that the Enabled checkbox is ticked. You then need to create a tag (using the Add tag button) to reference the script. In order for others to use the checker, the tag needs to also be active and refer to an existing cache.
The author of a checker can test it using an inactive tag refering to no cache at all, but no one else. You need to make a script inactive to be able to edit it. Don't forget to change it back afterwards.