How can I get my trackpad settings to work on the login screen?
Open Terminal and enter the following commands


defaults write com.apple.finder AppleShowAllFiles TRUE
killall Finder  




In Finder


open Macintosh HD/Library/Preferences
Ctl+click/right-click a file called .GlobalPreferences.plist and click 'Get Info'
Change the permissions to allow you to alter it but make a note/screenshot of what they are first. (be careful here!)
open the file by double-clicking  


  

Property List Editor will run and open the .plist file


Click 'Root' at the top of the window
Click the 'Add Child' button
Add an entry called com.apple.mouse.tapBehaviour
Make it a Number type
Set the value to 1
Save the file



Back in Finder, reset the permissions to what they were

In Terminal enter the following commands


defaults write com.apple.finder AppleShowAllFiles FALSE
killall Finder



Reboot and the next time you login you should be able to use Tap to click

If you run into issues with Terminal commands or with the plist file make sure your capitalization is correct, case matters!