Hello Roger,
I am not aware of any issue accessing the web interface of the SG200-26 from an OS X computer. If you were able to access it at one time, the IP address of the switch has possibly changed. It is possible the switch got a new IP address from the router.
Can you ping the IP address of the switch? If you can't ping the switch, check your router's DHCP client table to see if the router gave the switch an IP address. If that doesn't work, you can also try factory defaulting the switch.
Hope this helps.
-john