You don't need to go so far as to enter the DTE. The material editor is just fine for what you are trying to do. click the green button(labeled with a red arrow) to bring up the editor, and use the rotation controls(circled in red) to adjust the rotation of the texture. Also, if you want exact control with the rotation, you can click the degrees(the right side fo the rotation controls) and manually enter the amount you want(you can do this in numerous places throughout Bryce.

HTH
big_hoovie