AbleCommerce Shopping Cart Software

If I want to display a "Not In Stock" message for a Product (and disable "Add to Cart")
how can it be done?

I'm not using "Enable Inventory Management". Do I need to enable it?

Yes you need to enable to inventory management and then on same screen you can adjust Out of stock message textbox to you desired message. Finally you need to put some statement in AddToCart load method to hide the control if product.InStock value is zero.