I had this problem as well to fix this issue go to the terminal on your N900 and type root and hit enter and then type the following apt-get install -f and hit enter and type y and hit enter to the necessary applications or packages to install and now type apt-get install nitrdroid-installer and hit enter or if you have downloaded the package manually type cd and type in the directory where it is saved and hit enter.Then type dpkg -i nameofpackage.deb and hit enter.
Hopefully this should help