Hi,
So what this message is saying is that JSynthLib waited a while to receive some messages during the loopback test, but didn't receive anything. If you don't mind fiddling around a little bit maybe we can track down where the problem is.
Firstly I've just run the midi loopback test on my PC to check that it does work - in at least one case ;). The loopback test worked which proves its possible although as I'm running windows it doesn't shed too much light on your problem.
We can be sure that your problem is caused either by JSynthLib failing to transmit midi data, or by somehow failing to receive it. Perhaps we can start by trying to work out which one of these problems we are having. I suspect that as you are able to use the play sound button, the problem is not with sending midi data but lets be thorough and verify this.
Is it possible to check that JSynthLib transmits something when you initiate the loopback test? For example, on my interface it flashes a light every time midi data is transmitted or received. Maybe there is some midi software on mac that can monitor midi ports, or perhaps you could connect your midi port to your xp machine and use midi-ox or similar on the xp machine?
I'll have a think about ways we can test the receiving side of JSynthLib.
frankie