A complete new install of lubuntu 11.10 [...17.10] has synaptic 0.75.2ubuntu7 [...0.84.2]. Previously when using synaptic on another system there was a search box in the tool bar where a search string could be entered which would reduce the displayed packages as the string was being entered. The box is not being displayed nor do I find any setting to enable/disable it.
sudo apt-get install apt-xapian-index sudo update-apt-xapian-index -vf
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.