I'm pretty late on this.
You have two choices. One is what Tom described. The other one is to select an effect in the effect browser then right click and choose 'open a copy in motion' (if it's a basic effect which comes with FCPX). This will immediately add it to your library and you can do your changes in Motion and save them.
What I don't like: it can really blow up the amount of filters in your installation and your library which can make the workflow a bit confusing.
Andreas