The notepad I use does not list innerHTML as JavaScript... It maybe depreciated. I know its works for me because I have used it multiple times. Please list any errors or problems that you might be having.
Please show us your code so that we can help. Also, since it is a WP plugin, I would suggest you to have a look at jquery options too. There is a perfect method for this. val(). I also got a trouble with innerHTML while the val() method was a piece of cake.