Using mouse pad to zoom

Hi, I started using Lightroom a couple of months ago. At the moment I am just sorting my files (star rating and deleting the junk). I had a problem with my laptop and had to factory reset it (it's ok, all my photos were backed up!). My question is, when I was using Lightroom before, if I clicked on a picture and held down, I was able to use the mouse pad to zoom. Now all I can do is click on the picture and it zooms to what ever I have the zoom set to. Does anyone know how I can use the mouse pad again? It gives so much more control.