Admin, does it mean I need to make some configuration in pianoteq firstly according to what you mentioned and then I can use it in csound?

Yes that will be required if you want to control exactly the fundamental frequency of the notes played via pianoteq. But I suggest that you do not bother with that until you get pianoteq running in csound and can trigger notes on it via the vstnote command.

Admin, does it mean I need to make some configuration in pianoteq firstly according to what you mentioned and then I can use it in csound?

I try the vstnote example in the manual, but I can not successfully use vstplugg, the compiling result shows using vstplugg is invalid. however I use the newest csound to compile and vst4cs.dll has been included within the csound folder. I am so confused about that.

You will probably get more help from the CSound forums as your question is quite generic (using a VST synth in csound). According to csound doc you can use  http://www.csounds.com/manual/html/vstnote.html in order to send notes into the vst instrument. In order to have pianoteq generate sound for a given pitch, you should select the "flat" tuning in pianoteq , set octave stretching to 1, and then the midi note N will have a fundamental of freq 440 * 2^((N-69)/12). Using pitchbend midi messages, you can alter those frequencies, the default pitchbend range in pianoteq is +-200 cents.

Anyway who can tell me how can I transfer frequency value to MIDI note value in Csound. Many thanks.