There should be various "Set_Length" macros defined in topas.inc (around line 1080 in my version). I've pasted one below. Have you got the correct capitalisation? All the macro is doing is defining a rigid body containing the two atoms separated by r and allowing it to rotate around the coordinates of one atom, so you could always use that definition instead of the macro.
macro Set_Length(s0, s1, r, xc, yc, zc, cva, cvb)
{
rigid
Point_for_site(s0, 0, 0, 0)
Point_for_site(s1, 0, 0, r)
Rotate_about_axies(cva, cvb)
Translate_with_site_start_values(s0, xc, yc, zc)
}