Error trapping on field in Zen Cart input form (PHP)

I am running an online store called scrapbookersplayground using Zen cart with a modified script. This is a script that allows for the input of information regarding downloadable digital scrapbook products, and is used by multiple sellers. There is a field for putting the name of the zip file. Many times they neglect to fill in this field, or do not put the .zip on the end. I need this field to requile these things before processing the submit button. This should be less than 30 mins work for anyone who knows Zen cart. Please indicate in your responses whether you are familiar with Zen cart, and do not ive me canned responses or long responses telling me your whole set of qualifications, These will be ignored. Only those who address my specific requirements for this particular project will even be considered. Good communication skills are also a plus, as there will be a good probability of future work being required. Thanks, Katrina Haney