I want my Nexus 7 to know when I'm leaving for work and display the details about traffic and such. But I don't know where I need to add home and commute destination.
Actually you shoud set you home and work address in Google Maps/Latitude. You can do this at least on your desktop by going to http://maps.google.com, logging in with your Google account, and clicking
My places on the left. This shows the fields where you can edit your addresses.
Google Now will figure out your regular schedule after a while, took a couple of days for me to get the suggestions at the right times.