How to install deb files on Harmattan
Harmattan security doesn't allow users and developers to install debian packages on the device using the terminal. On Fremantle/Nokia N900 install deb packages was simple since it was easy to root the device itself. Things are slightly changed on Harmattan because security has been increased, this is why "dpkg -i package.deb" cannot run.
To workaround this, Nokia provided developers with MADDE tools which can be installed on the developer device by simply setting the device in "Developer Mode" (Settings > Security > Developer Mode).
Madde tools package ships devrootsh utility, that allows developers to run dpkg with root privileges.
How to install a debian file?
To install debian files on your harmattan device the command to use is:
/home/developer $ /usr/lib/mad-developer/devrootsh dpkg -i *.deb
If it worked out you will see messages like this: aegis-installing xxxx (from 'com.nokia.maemo')