Fresh install of lubuntu synaptic is missing the search box in the tool bar
sudo apt-get install apt-xapian-index
sudo update-apt-xapian-index -vf
From: http://ubuntuforums.org/showthread.php?t=1178974
If the other answers don't help then try:
sudo apt-get install --reinstall synaptic
sudo dpkg-reconfigure synaptic
It worked for me.
forums linuxmint suggests installing apt-xapian-index when the box is present but grayed out. This solution also works in this case where the Quick Filter box is completely missing.