The problem you've got here is that Sketchup creates a mesh model and Inventor Creates a solid model. Even if you get the Mesh into Inventor it will appear 'Jagged' - Circles and arcs will look like polygons and faces will be composed of a number of triangles.
Check Out this post for an explanation of the differences:
You could try exporting to SAT, which Inventor should read.
You could try this download from Sycode:
(Includes a good graphic that shows what you are going to get)
You would probably be better off re-modelling the part in Inventor.