There are several network simulators available which are design for different purposes. Quick google will result survey papers discussing these simulator both advantages, disadvantages and usage areas. After deep diving into these simulator environments, OMNeT++ is the best logical option for simulating WSN (Wireless Sensor Networks). OMNeT++ is available as commercial and academic uses and actively developed.
Also, OMNeT++ provides infrastructure to develop components top off on it and WSN is available as 3rd party component. OMNeT++ models are available in its website. Mixim is a modeling framework for mobility and fixed networks which is necessary simulating wireless networks with OMNeT++.
OMNeT++ 4.2.1 and MiXiM 2.2.1 is greatly integrated and works well under Mac OS X Lion. After installing OMNeT++ MiXiM is imported into workspace and compiled under OMNeT++. To be able to compile and build tools latest XCode version is needed.