Like everyone is saying Easel is a great tool to get started. But if you are serious about making great designs buy Vcarve Desktop today (you can upgrade to Pro later). The Xcarve is a very capable machine and can do amazing things, but without powerful design software the entire process of designing and carving can be very frustrating.
Save yourself months of that frustration and get the right tools for the job. I know that $349 sounds like a lot of money, but trust me, you will be able to make products that will pay you back in less than 3 months without even trying. I never intended to sell what I made, but just by making things and giving them away free to friends and family I received orders for more stuff than I could make in my spare time. So think of buying the software as an investment that will pay for itself many times over.