Got one requirement to use Actions and tried to Register Action using Plugin Registration Tool.

But Action messages are not listed and done the below things,

1. Whether Action is Activated or Not. Action is Activated.
2. In plugin Registration Tool,
   1. Reloaded Organizations &
   2. Refreshed Messages and Steps.

Finally I Closed and Opened Plugin Registration Tool. Then Action Messages listed in Plugin Registration Tool.

Hope it helps.