Some scholars would actually argue that the media influences audiences away from participation. They would say that the media shows the bad side of politicians to such a large degree that people become disgusted and are less likely to want to participate.
If, however, you are supposed to argue that the media encourages participation, you can say that it is because the media lets people know about the problems that are going on in the country. This, for example, is what happened in the Progressive Era when the muckrakers had such an important influence. When people find out about these problems, they become motivated to act. They participate politically in an attempt to push the government to solve the problems they see portrayed in the media.