Select Effect Titlebar to Display On-screen Controls

When working with an effect in Final Cut Pro, make sure the effect titlebar is selected in order to display any on-screen controls related to the effect:
1) Select the clip on the timeline, and make sure the current time indicator is sitting within the clip.
2) Switch to the parameter inspector to display a list of all effects applied to the clip, along with their parameters.
3) To display the on-screen controls for any given effect, select its titlebar in the parameter inspector:

Final Cut Pro only displays on-screen controls for the currently selected effect. This allows you to apply multiple effects to the same clip, while being able to access on-screen controls for one effect at a time.