Apple Motion 3 - Manual (Page 611)

Chapter 7 Using Text 611
4 To position the texture, do one of the following:
 Press Command, then drag in the Image well.
 Use the Offset sliders or value sliders to enter an offset value.
The image within the text moves, and a keyframe is created in the Offset parameters.
5 Go to the next frame where you want to set a keyframe.
6 Move the texture to the new position.
Text layer with texture applied
Repositioned text layer texture