hi. I just followed the tut to ensure I could duplicate it, and I can, so we just need to figure out where you got stuck. Can you attach the file so we can see where it went off course?
If this helps any, you might have been successful but just need to move the handle to adjust the design. After you draw the arc, you need to go into the node tool to be able to do that. You might just need to grab the handle from the circle to make the design larger (I needed to make mine bigger). Other possibilities off the top of my head could be that your nodes need to be auto sooth, or you may need to highlight all the nodes, click the + and add more nodes. Working with Spiro can go wonky very fast, I find that adding more nodes usually helps. Also ensure that your 'smooth' number is around 42 (what the author used), but I went a bit lower.