I am now spending time on trying to learn this program with the free trial period which has only two days left. I find it tricky and difficult to use. After some work I have come so far as to have ATC answer my requests. For example altimeter setting. But the answer is really weird 0880 and not correct.
Then I ask for VFR clearance and was told to 

Repeat your request with a proper call sign Cessna Two Sierra Papa

my call sign is Cessna N172SP . Why is that not proper ? It is a Cessna 172 and N172SP is written on the plane. How can I know if a call sign is valid ?

Assuming you have the Call Sign set up as a GA call sign, aircraft = Cessna and Tail Number = N172SP, then Cessna Two Sierra Papa is a valid call sign.

It's possible that the program is not hearing and recognizing the correct call sign.  Look at the Green text in the Speech text window (Bottom Right of the main screen) to see what the program is hearing.

If it looks right there, send me a screenshot of your main screen with the interaction visible and also one of your setup main tab with the call sign setup to [email protected].  

Dave

2 hours ago, Dave-Pilot2ATC said:

Assuming you have the Call Sign set up as a GA call sign, aircraft = Cessna and Tail Number = N172SP, then Cessna Two Sierra Papa is a valid call sign.

It's possible that the program is not hearing and recognizing the correct call sign.  Look at the Green text in the Speech text window (Bottom Right of the main screen) to see what the program is hearing.

If it looks right there, send me a screenshot of your main screen with the interaction visible and also one of your setup main tab with the call sign setup to [email protected].  

Dave

From what I think I remember the green text has at least sometimes displayed what I said correctly. Also I have experienced that my requests has been correctly heard but still ATC does not answer. What could that be ?

Be sure your radio is powered on and you are tuned to a valid controller frequency.  If not, then your speech may be recognized, but you will not get a response because there is no Air Traffic Controller on the radio to respond to you.  As in the real world, you will just get silence.

There are also cases where you are tuned to a controller, but that controller cannot handle your request.  In most cases they will  give you a frequency change to the right controller, but sometimes there will just be dead silence.

Dave