In recent years with the rapid growth of wireless devices, Mobile Ad hoc Networks (MANETs) have become a very important technology. However, MANETs are more vulnerable than wired and wireless networks. Whereas, MANETs are deployed in un-trusted environments, so, security and intrusion detection are very important and one of critical issues in these networks. In this paper, we present a scheme for determining of IDS agent nodes with applying a three-tiered architectural framework. Our scheme selects the nodes of the network which are valid nodes and also have higher battery power. IDS agent nodes because of monitoring activities in Intrusion Detection System (IDS) need to more energy than other nodes in the network. In the first phase, our scheme uses three-tiered key management framework (DDC) in order to authentication. So, authorized nodes contribute in monitoring activities and also communication cost in three-tiered key management framework is less than other frameworks. In the second phase, IDS agent nodes will be selected from among valid nodes which have higher battery power. Therefore, network lifetime will be increased. [ABSTRACT FROM AUTHOR]