Juniperus horizontalis has been cultivated primarily for landscape uses. None of my books say anything about the berries being edible, but seeing as how its primary use is that of an ornamental groundcover I'd say its probably not the best juniper to use for berry harvesting.
edit: Looking at a few more books in our library (i'm bored at work today) it says that the berries can be roasted and made in to a coffee-like beverage. The leaves can be made in to a tea. Having said that, I'd venture to say that most cultivars that have been nursery grown will not have as great of edible characteristics as the ones you would find growing in the wild. I'm thinking that they are not native to Oklahoma and any you would find growing will have been nursery grown or escaped cultivation.
Reason: further research