Pianoteq in FL studio

I am not sure what answer you are looking for, exactly. I don't have FL myself (I use Sonar and Reaper), but it seems that FL can use VST plugins. Pianoteq is a virtual instrument, and it includes a VST plugin to be used in a DAW.

So the general idea would be: set up the correct paths in FL so that it can find the Pianoteq VST; then let the DAW scan for new plugins (if that's not done automatically). Finally you usually insert an instrument track in your project, selecting the Pianoteq VST. Sometimes MIDI routing must be configured as well, but usually the default is omni-input.