Hi Aurelia, I've been having similar problems. I'm totally new to mesh creation and so it's very possible that I'm doing something wrong, though I did read the wiki and some tutorials on the appropriate steps for uploading.
What's happening to me is that when I hit the "calculate" button, the process just stalls completely. I was able to successfully upload 1 model yesterday, but that was it. Since then, every time I hit that "calculate" button, it stalls. I've waited up to half an hour with no progress.
I kinda gave up this morning and hopped over to the forums to see if anyone else was having these problems.