If your version of FL Studio supports virtual instruments, these are the typical steps to use Pianissimo:
1. Click on the Channels button, and select Add One->More..
2. A list of DXi and VSTi plugins, called the "Select generator plugin" window, will be displayed. Scroll down to the VSTi section and find Pianissimo in the alphabetized list. Click on the box next to Pianissimo until it is "checked" (a small F will appear inside the box when it is checked). Now close the "Select generator plugin" window.
3. Click on the Channels button, and select Add One->Pianissimo.
4. A Pianissimo track has been added to your project, and the Pianissimo editor window will be displayed. You can now use Pianissimo as you would use any other generator in FL Studio.