The changes to the files are for Zen Cart version 1.3.8a and may be different for your system depending on what version you are running.

For this change we will be editing the core Zen Cart files and since there are no overrides for the admin area as discussed in this article, be sure to make backups and to remake the changes if you upgrade or overwrite the core files for whatever reason.

In the the file:
[admin]/includes/modules/category_product_listing.php
using the name you used for your Zen Cart admin directory in place of [admin] (you DID rename your Zen Cart admin directory, right!?).

At around line 843, the function zen_draw_pull_down_menu is called with two parameters:

zen_draw_pull_down_menu('product_type', $product_restrict_types_array);

Using phpMyAdmin or some other way of accessing the zen database, browse the [zen_prefix]product_types and get the value for the type_id of the product type that you want to set as the default. For this example, we want the ‘Product – Music’ to be default which has the type_id=2. Now in the category_products_listing.php file, add a comma and the type_id value like so:

zen_draw_pull_down_menu('product_type', $product_restrict_types_array,2);

And your done! Now you can add ‘Product – Music’ types without the step of selecting the type. Also, if you wanted to add products of other types as well, just change the selection in the drop down.