Hi,
Additional packages can be added using apt-get, which is a command line tool. The synaptic package manager is the GUI wrapper around this, but I am not sure how well it runs on the PI. To update your Raspbian installation, open a command window and type
sudo apt-get update
sudo apt-get upgrade
To install more packages, type
sudo apt-get install package-name
where package-name should be replaced with the package you are interested in. The full package list for Wheezy can be found at,
http://packages.debian.org/testing/
The other apt-get commands are documented in the apt-get manual page,
man apt-get
(type q to quit the manual page).
Regards, Will