I found that on my Mac, the curfew imposed on my son's account can be bypassed simply by clicking on 'OK' button of [use limit nearing] notification or just login again after force logout. But NSM works perfectly on my PC without hassle. Even uninstall and reinstall NSM on my Mac did not solve the problem.
system info:
iMac 27" late 2009
Macbook mid 2010
OS 10.6.4
NSM 1.3.1f107