Put a Transform Shader after/below your cloud Density Fractal (before it goes into the Cloud Shader's density shader input), then use the Transform function to move the noise function. The units are meters, so you're effectively moving the clouds (or rather the noise function that creates the cloud shape) across the sky by n number of meters in each dimension. It's X, Y, Z coordinates, so to move horizontally you'd use X or Z as Y is "up" in Terragen.
- Oshyan