First you'll need to uninstall whatever Plastic SCM version installed on your machine.
Then follow the instructions to get Plastic SCM installed using the newest packages system. Don't worry about the "Debian_6.0" name, it works fine with the 7 release.
Add our public repository to your APT sources list:
echo "deb http://www.plasticscm.com/plasticrepo/4.1/Debian_6.0/ ./" >> /etc/apt/sources.list
Add the key to the repository into APT
wget http://www.plasticscm.com/plasticrepo/4.1/Debian_6.0/Release.key apt-key add Release.key
Refresh the APT cache
apt-get update
Install PlasticSCM
apt-get install plasticscm-client plasticscm-server
If you have any question don't hesitate to use the Plastic SCM forum -> www.plasticscm.net