Mixcraft will automatically find any new plug-ins, including Pianissimo, when Mixcraft is first run after installing Pianissimo. Run Mixcraft, and add a Virtual Instrument track. Click on the Instrument button on the virtual instrument track - it's in the shape of a small piano keyboard - to bring up the Instrument Dialog.
Now, click on the Instruments header to see a list of available plug-in instruments. Pianissimo should show up as an available virtual instrument in your DAW.
Select Pianissimo. It may take up to a minute to load Pianissimo's sample data, depending on the speed of your computer. A Pianissimo loading screen will display during this time. Once the loading is complete, press the Edit button to open the Editor window.
All of Pianissimo's controls will now be available. You can lower and raise the piano lid, adjust the tone controls, change the reverb amount and type, and more. Use your DAW host to play back a MIDI file, or control the instrument via your MIDI keyboard controller. You can also click on the virtual keys to play the piano sound. Try out the different presets, and have fun with it!