Using the Log function in MBS Plugin you can write log messages like this:
MBS("Log"; "Hello World")
On OS X you see those messages in the Console.app. For Windows, you can install Microsoft's DebugView application and see also the messages.
This even works fine on the server. On Windows with a FileMaker Server, you can see debug messages in DebugView if you run DebugView application as administrator and enable global Win32 listening.
For FileMaker Server on Mac OS X you can see messages in /Library/FileMaker Server/Logs/stderr file. This is just a text file and you can open it with Console.app to show you incoming log entries.
If you run MBS("Trace"), all plugin calls will also be logged there. This enables you to debug plugin usage by checking if all function calls have right parameter and expected results.