What exactly are you trying to accomplish?

Where do you want to be printed?
I would suggest to have a button with each row that has that "onclick" event.
When the rows are dynamically created you can have the value you want to be printed...

I usually use javascript to manipulate the state and value of the input fields. Haven't combined it with tables.
But I would suggest to post the code you have for the table as well as the javascript...

The "mainMenu" should be the id of the table.
Since you are using this:
document.getElementById("mainMenu")

It is quite obvious when you say "getElementById" that the argument should be the id...