If you're wanting to add your daily weight to your Daily Entry, where you list your foods, I don't think that's possible. I could easily be wrong, though.

You can create a graph of your weight by going to My P.L.A.N. and clicking on Progress & Charts in the menu on the left. Across the top of that page, there are a number of links you can click on to add stats for various purposes.

I will say that, IMO, the weight graph gets really messy looking if you record your weight there daily. It might be okay for you, though. Another option would be to record your daily weight as a daily note. To do that, just click on View Notes in the menu on the left.

I find it simplest just to record my daily weight in my journal, and then record it on my Weight Graph once a week.