Log into your Mals account and click on Cart Setup. Then click on Other Messages & General Settings.
You will see a box under the text that says "CUSTOMER MESSAGE. If you want to allow your customers to include a message along with their order then enter a description of its purpose for the customer. Fill in that box with the word Comments or Questions or whatever you prefer it to say. Then click update.
Since this box takes up a bit of width in the cart, you may want to set your cart to display in a new window vs. inline frame (within your website page). To do that, log into your storeadmin and click on Catalog Setup and set the Order Process to Normal Window instead of Inline Frame. Then click Update Variables.