I loaded separate protein models as requested by Chimera. One was a peptide I obtained by deletion of the coordinates of the polypeptide chain to which it was bound and changed its chain ID to "B", using Biovia Discovery Studio VisualizerTM. I then renumbered it from 1-12 to 985-996, using Win CootTM. The other protein model I loaded was a model of a protein that I had renumbered from 1-176 to 888-1063, using Win CootTM , following which I had renamed the chain ID to "B" (using Discovery Studio) and then deleted residues 985-996, using Chimera and saved it as a separate PDB file. After loading both the models in Chimera, I had selected the C-terminal C atom of the peptide chain (of residue 996) and the N-terminal N atom of the latter part/fragment of the larger model (residue 997), which had residues 985-996 deleted from it previously. Following this, I clicked on the following chain of buttons to join the two models using the selected N and C atoms: Tools-->Structure Editing -->Build Structure-->Join Models. However, the final "Apply" button in the Build Structure dialog box remained disabled. How do I enable it so that I may insert this peptide into the middle of my larger model, using the Join Models option?

Note: I even tried the above experiment with the coordinates of the peptide and those of the C-terminal portion of the larger protein model (residues 997-1063, saved as a separate PDB), opened in Chimera as two separate models. It still didn't work! :( :( :( ... PLEASE HELP!