add a jar to my project if using the jar get CLASSNOTFOUNDEXCEPTION

unity3d 4.0.1 create eclipse project. in eclipse,put the jar to libs,add jar. in ProxyActivity i use the jar ,I will get CLASS NOT FOUND EXCEPTION. if i create a new activity in the project Unity created and use the jar ,I get the same Exception. if i create a project in eclipse and add the jar ,its all right.