DoD doesn't make it easy for retirees to make an appointment online if you're not sitting in front of a USCG or DoD computer.  This procedure installs the DoD Security Certificate needed to access the online appointment system from a civillian PC, through either of the following links:
Base Seattle:
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First,  Determine whether your PC is a 32-bit or 64-bit machine.  In Windows 10, right click the Start Menu (Windows icon at bottom left of the task bar), left-click "System," and make a note of "System type," either 32-bit or 64-bit..

Second, click on the appropriate link, depending on a 32-bit or 64-bit system:


Save the appropriate file to your "Downloads" folder.  Then run it and follow the prompts.  It should show "Install DoD Certficates" as the only selectable option for installing the certificate.  Go ahead and do that.
After installation, click on one of the Web sites above.  You should no longer get any warning messages on your browser.