I am Croatian, but I have had experience with getting visas for USA and UK. Be sure to have all your documents and also bring a photo of yourself that is not more than 6 months old (they really gave me a hard time because of that!). Other than that, I think you`ll do fine.
I hope you get the visa and enjoy your stay in Croatia!
Ivana

Going there with all your documents is a good idea. (until then) I would keep trying to call them and get thru.
The few times i had to get a document like that, it went smoothly. Good luck with getting your VISA.