I can log it in Classic & 2.0, I'm not sure if it's limited to premium members or not.
In Classic, click on "stats" and it should be there (along with a ton of other stuff you can track).
To try it in 2.0, try this:
Go to the "profile" tab,scroll down to "Preferred Theme", change to "FitDay 2.0" and click save.
Now click go "Log" > click "Custom". That's where it would be if you can do it.