(have you tried turning it off and on again?)
And have you tried to delete Winbox files (from Application data)? - maybe the plugin is invalid...
Do other services (telnet/ssh/webfig) work normally?
edit: wait, just let's go back to the basics
You have just connected the RouterBoard to your PC (or network), right? And WinBox shows it on a list - and you try to connect with MAC address? Maybe communication using MAC is failing - and you need to try to connect with IP.
If it's the case - just try to set a 192.168.88.2/24 address on your computer - and then connect with the RB using the IP.
Or it can be the case, when WinBox shows the routerboard, but without any IP - maybe it had a reset with "no-defaults", and it doesn't have 192.168.88.1 address at all?