The complete question: "I need information about the following two datablocks:
Background information: for using the macro build_alpha_beta.odb I would like to use only the FRAG_ROT dials when I add a new piece of helix. For this to work, the pivot point should be in the first Calpha of the new piece."
If you want a particular atom to be the pivot for move_zone, double-click it (i.e., this only works when the zone is one residue !) The id of the last picked atom is in .symbols; if you centre on it (even if only temporarily) the coords should be in .active_centre. You may want to check the macro file://rose.bmc.uu.se/pub/gerard/omac/move_to_origin.omac which shows you how to move a molecule to the origin at (0,0,0).
By the way - with the new baton commands in 5.10 you won't need the build_alpha_beta.odb macros any longer since they allow you to automatically build helix or strand.