Read this, it's important! NEW CURA DEVELOPMENT IS HAPPENING AT https://github.com/Ultimaker/Cura, this is the Cura 15.04 archive. Cura 2.1 and newer is on the Ultimaker github.
This sounds like the export from inventor pro is wrong. If it changes circles into hexagons, it's usually the resolution of the exporting that is too low.
When I download an Stl from inventor Pro it doesn't come out the correct size and all circles come out hexagons in Cura