I got a new ENO Guardian net a couple of months ago. The very same thing w/mine! Just zipper "stock" sewn in from top to bottom w/no tongue or the thing the tongue attaches to either. (I don't know the correct term for all these zipper parts!) That's why you couldn't open it with needle nose pliers or small string, etc. There was nothing to grab!!
I called customer service. They were very nice. I sent it to them and they promptly sent me another one. I got the original one off ebay.
The second one is perfect. I like to way it opens vertically. When it's unzipped, it's not hanging wide open to allow bugs a bigger target to enter.