Chen,
Do you have a valid license installed?
Check this by opening GEP_INSTALL_DIR\data\log\karaf.log and looking for a line the says "LicenseChecker." (GEP_INSTALL_DIR defaults to ARCGIS_SERVER_INSTALLDR\GeoEventProcessor. ARCGIS_SERVER_INSTALLDIR defaults to C:\Program Files\ArcGIS\Server. So for most installations, GEP_INSTALL_DIR= C:\Program Files\ArcGIS\Server\GeoEventProcessor)
If you have a valid license, the line will say "License status: GeoEventProcessor: licenseStatusSuccess"
If it doesn't say that, you'll need to install a valid license. You can do that by following steps 1-6 on this site.
Let us know if this if this helps you out.
-Javier