Hi riko - generally you will want to focus on making an initial surface that's more like square sheet and then using booleans or trimming to cut the outside of it off to make the actual rounded boundary edges.

See these threads for some descriptions:
http://moi3d.com/forum/index.php?webtag=MOI&msg=2021.1
http://moi3d.com/forum/index.php?webtag=MOI&msg=3618.1

- Michael