Opioid receptors, delta, kappa, and mu

If you can somehow get the PDB representation of the various GPCR component structures, you can view them in the application. Many of them should be in the RCSB Protein Data Bank, so you could search for them in that part of the application (note that I'm not affiliated with the PDB, so I don't have any say over what's in the database there).

If they're not in the database, you can still obtain the PDB files and place them in the application manually using iTunes file sharing. This can be found by connecting your device to iTunes, going to the device, selecting the Apps tab, and scrolling all the way down to the File Sharing section. In there, you can click on the Molecules icon and drag any PDB files you want into the right-hand list. The next time you start Molecules, these structures will be loaded into the application.