Mentalism refers to the fields within psychology in which inner motivations for behavior are emphasized as is the case with traditional therapies, such as in Gestalt, Rogerian or Jungian psychology. It also refers to emphasis placed on mental perception and cognition, such as in cognitive psychology.
Behavioralism is something completely different. A discipline that focuses on politics, the goal of behavioralism is to understand how people think and act, whether as individuals or part of a specific group. Researchers subsequently apply what they observe and measure from these individuals and/or groups to the current political climate so they can try to ascertain how these people will act/vote in elections and how they will act as pertains to politics in their jurisdictions as a whole.