I went and tested this to quite an end. First you should determine if your trackpad is actually working.
My steps were as follows to confirm:
1) Go to System Preferences --> Trackpad --> Scroll and Zoom
2) Confirm that Zoom in or out has a check mark.
3) Test another gesture such as scrolling.
4) Now open a JPG or PNG in Preview. Try the pinch and zoom.
4A) If it works, QUIT the application where it is not working and retry.
4B) If it doesnt work, then restart your machine
5) At RESTART, ZAP your PRAM! Command + Option + P + R
6) Open System Preferences --> Trackpad --> Scroll and Zoom
7) Uncheck your Zoom Preference
8) Click Show All or Close System Preferences so the TrackPad preference gets a fresh write with the setting unchecked
9) Go back to System Preferences --> Trackpad --> Scroll and Zoom
10) Check the Zoom feature
11) Test again.
In MY case I was done at step 4A. I was able to quit safari and mail and upon reopening the application I was back in business.
Hope this helps you all.