Nobody reported this and, in fact, the only comment we got on the new cargo stairs, is they look cool...
Try this: enable logging in the GSX Troubleshooting page, then restart Couatl (so it will read the new setting), and select either boarding or deboarding with the same cargo airplane, then just exit the sim.
Check the diagnostic log here:
%APPDATA%\Virtuali\Couatl.LOG
and look the last lines: they should hopefully tell something about why the stairs didn't appear.