Trouble adding rule to a variable product

Hi,

I want to add a validation rule to a variable product, basically I want customers to fill out a required text box, but I have no idea how to do this. I noticed in the Quantities and Units for WooCommerce plugin description that it says it can be applied to all variable products, but I’m not sure how since it seems you can only use tags and categories, which as far as I know isn’t available for variable products unless you do some hardcore coding. I’m pretty new at this so I could very well be missing something or misinterpreted the plugin description, which is very possible. Any help would be super appreciated.