At the current time, you cannot change the default location of the Workout Blender files once they have been set upon the first installation of the software. This is mainly because if the locations are changed, the software will lose track of all of your created workouts, the location of the original video files, and the videos that have been generated by the software. This is something that will change in future versions of the software, but for now if cannot be done without deleting the preferences file for the software (in which case all of your premixes created within the Blender would be lost from your current version).
If you would like to remove the preferences file (remember...everything you have done within the Blender so far WILL be lost, but you WILL NOT lose the original video files or the blended video files you have created...these will still be located in the folder you have currently assigned them to), please follow the instructions in this forum thread: http://cathe.com/forum/f122/users-80%-complete-videos-not-importing-problems-280095/
Upon launching the Workout Blender for the first time after deleting the preferences, it will ask you where you would like to store your files and videos and you can change the location at that point. From that point on, the videos will be stored in their new location. You can move the old videos you created from the old folder to the new one, also, but the Workout Blender will not acknowledge their existence since the old preferences are gone. They will still work fine, though, in iTunes, video players, iPods, etc.