Guys, I imported some MIDIS files directly from Guitar Pro for Sonar x3 . The intention is to use them together with voice and guitar recordings. However, when I export the project to WAV, MIDI files are not exported together. I've tried select them and click track> bouce to track , but it appears that:

"The specified selection did not contain any audio data. Please ensure that the selection contains the audio data you wish to bounce."

I've tried using the " freeze " , but this option is not selectable , is gray. Looking on the internet I found that I would have to use the synth rack for this, but I have no idea how to use it .

In short, I want to turn the MIDI tracks to audio , to export all then WAV . Can someone help me?

( Excuse me if there are errors , I am using a translator )

Welcome to the forum!
What is producing the sound for the MIDI tracks?
You should have a soft synth inserted in order to bounce/export/freeze MIDI tracks as audio. In case you have inserted a soft synth with separate MIDI and audio tracks, both of them must be selected for bounce or export.
Maybe you are using the MS Wavetable synth, which is by far the poorest option soundwise, and also, it can not be exported the normal way.
 
It used to be tutorial #4 that explained the use of soft synths. The Help files is the closest place to look for answers but there are also several tutorial videos in YouTube. I've never watched any, so I don't know exactly where to find the best ones.
If you click Insert>Soft Synth>TTS-1 and select "First audio output" you get automatically the MIDI track and audio output track. TTS-1 can handle 4 separate MIDI tracks with different instruments, so you can also check "All audio outputs" if you have several MIDI tracks.

On your midi track, what is showing as "output" port/synth? 
Midi is just note data, like key x is pressed with velocity y at time z. It doesn't contain any audio at all. In order to hear it you must have it assigned to a hardware or software synthesizer. E.g. Cakewalk supplies the TTS-1 synth for a basic set of sounds that follow the General Midi standard.