This posting is older than 6 months and can contain outdated information.
Thanks for asking.
Custom actions can be mapped, but not be created or edited in global mappings. For this reason, the "custom actions" item is only available for Behaviours.
To edit your custom action, change to Prefs > Mapping > [App you've added the custom action to] and choose "Custom actions.." from any popup there.
Best regards,
Felix Schwarz