Your code looks good, but as William points out the error indicates you aren't using the required version of the jdk. The supported version message of 48 indicates jdk 1.4 or higher is required. I believe jamon could be recompiled to work with 1.3 though I haven' tried that. It requires 1.3 or higher due to its use of the recently added proxy class.
The code you use will track performance statistitics for any Java interface, by adding that one line of code (A obj = (A) MonProxyFactory.monitor(new A())). If you display the jamon report you will see information about how many times your method was called and execution stats like avg/min/max/total among other things. You could print the jamon stats with the following line: System.out.println(MonitorFactory.getReport());
Or you could run your test code in a web app and use the jamon war viewer.
Also I suggest in a production app to hide all jamon code in a utilities class like the following: