You have 3 ways to start an effect:
- Time base
- On click of an interactive object like a (shape) button
- By advanced/shared action
Have a look at the movie in my first review article of Captivate 9:
Playing with Captivate 9 - Captivate blog
The custom curved motion effects are applied by shared action, in the same action that displays a state for the central button. There are a lot more possibilities with those actions than by clicking a button. But if you want to use just a button: in the dropdown list for the Success event of a button you'll see 'Apply Effect'. If you choose that command, you'll be able to indicate to which object the effect has to be applied and to set up that effect.
BTW effects are not new, with the exception of the curved motions and the scribble path effect. The work flow has been improved a lot. About 4 years ago I wrote this article:
Editing Motion Paths and Reusing Customized Effects - Captivate blog
It is still visited today.