Are you using the standalone version of Pianissimo, or are you using it as a plugin in another program?
If you're using the standalone, click on the File menu and select Preferences. If ASIO or WaveRT are available, try one of those modes. If you're already using WaveRT and ASIO is not available, you can try reducing the latency setting. If you don't have an ASIO option, you can also try downloading and installing ASIO4All