Follow the steps described in this article if you want to force a checkbox to always be checked regardless of the database value - for instance, an email opt-in checkbox.
Get the field's ID
First, get the HTML ID for the form field you want to edit. In Design Studio, select a landing page that contains the form and preview the page.
View the HTML source code of that page and find the field you want. The fastest way is to search for the label that you used when you created the form, such as "Opt-In".
Search for the "id" attribute in the "input" tag for that field. Below, the id is "Opt-In".
<label>First Name:</label><span class='mktInput'><input class='mktFormCheckbox' name='Opt-In' id='Opt-In' type='checkbox' value='1' tabIndex='1' /><span class='mktFormMsg'></span>
Setting the checkbox
Change the highlighted yellow bits below with the name of the field.
// use no conflict mode for jQuery
var $jQ = jQuery.noConflict();
// when the page is ready, change Opt-In