Apologies for the very late reply. First login to
https://pushover.net/ with your account, and select 'Register an Application' - you can set the type to Plugin and it doesn't need a URL (just a name and description).
This will give you an API key with 7500 monthly calls for free. This API key goes in the token config box in the plugin.
Next go to
https://pushover.net/ and copy your User Hash and enter that into the plugin as well and you're all set.
PM me if you have any issues.