This is not an issue that I can reproduce. Please post back and include the specific steps that you are taking when adding this coupon to the store. Also post back with info about the result that you are getting.
When I try this I'm able to add a coupon to specific items by checking the box for choose items, then when I add the coupon I'm taken to the product search page where I can search for any of the products, then on the results page I can add individual items to the coupon. What do you experience when you do it like this?