there's no magic involved, if your instalation fails it must be because the UID used in the packages you sign are not assigned for use with your devcert (i.e. you don't own them) or you don't have the required capabilities in your devcert. Since the UID used here is in the 0xE range the UID itself should not be a problem. So it must be the capability.
Use the DevCertList tool ( http://projects.developer.nokia.com/DevCertList ) to analyze your certificate and make sure it has the capabilities required (you can find them in the sourcem the keypress.mmp file)
CAPABILITY PowerMgmt ReadDeviceData WriteDeviceData TrustedUI ProtServ SwEvent NetworkServices LocalServices ReadUserData WriteUserData Location SurroundingsDD UserEnvironment