The sum of a continuous signal is the integration.  So, you can use Modelica.Blocks.Continuous.Integrator to get that sum.  Then, divide by t to get the average.

Thank you, dersh. But it gives me time varying data, but a want a constant single value. How can I obtain that?

The mean of a time varying signal is going to be time varying.  So, I really don't understand what you are looking for.  Can you explain more of what you want?

Yes you are right, but I want to take samples from the time variyng signal for example: t=0 x(t)=0, t=1 x(t)=1, t=2 x(t)=2 and the mean value is (0+1+2)/3=1. How can I obtain the constant mean value?

You can take your continuous signal, then pass it through Modelica.Blocks.Continuous.Sampler then integrate it.  Between each sample the value will be constant, but the integral will increase.  But, when you divide by the time, you should be able to correct that out.  You can also then use sampler again after, if you want it not to change. 

Here's an example of the sum of a random variable:

sampling-test.mo

Thank you dersh