You mean you cannot type in the search field of Synaptic?
This has happened to me as well a few times, just try to close it and open it again.
In case you have to luck open a terrminal and we will use nano as editor to edit your sources.list file:
sudo nano /etc/apt/sources.list
Check first if the lines are already there:
# Debian Squeeze/6.0
deb http://ftp.us.debian.org/debian squeeze main contrib non-free
If there are there but the line begins with # means that the are commented so remove # form the line that starts with deb.
If lines are missing totally then add them by copy/paste. Keep in mind that in terminal and nano we copy/paste with Shift/Ctrl and then c or v respectively.
After that press Ctrl/o to save the file and Ctrl/x to exit the nano editor.
Then in the terminal run:
sudo apt-get update
sudo apt-get install firmware-brcm80211 wireless-tools
The above will install the necessary broadcom firmware that most probably is not presen tin your system. After isntalling try to see if you get a wireless interface and report back any truble you get in the way.
Alternatively if you get o fix Synaptic you can perform the same actions through the safety of a GUI;)
If you break it, you get to keep all pieces.