Whilst Seiko's Smart Label Printer software (v1.8.2) continues to run with El Capitan (MacOSX 10.11.3), it has become unusable because a scaling issue has somehow crept in, and labels are not printed at the correct size (they are much too large). I have contacted Sekio support and they simply agree that it does not work anymore and say they do not plan to fix this relatively trivial problem. They suggest using their new Smart Label Creator software instead, although that does not work with the SLP 430 printer I have.
Furthermore it is obviously a pretty horrible direct port of their Windows software and looks like it with ugly Windows-style icons. What's more there is no ability to access the Mac Address Book, so what with one thing and another I have now jumped ship to Dymo, whose software is not perfect but it is at least vastly better than the new Seiko software!