Just as in the prompt and scripting UI modes, the molecule, residue and spin data structures need to be set up prior to the loading of any spin specific data. The ``Spin systems'' GUI element is used for this purpose. Before any spin systems have been set up, this should say something like ``0 spins loaded and selected''. To fix this, click on the ``Spin editor'' button and you should then see the spin viewer window. The next steps are fully described in section 4.5.2 on page for PDB files or section 4.5.3 on page for a sequence file. The spin viewer window can now be closed.
The relax user manual (PDF), created 2016-10-28.