You will need to save it as an Illustrator 8 EPS file.
How to save to an Illustrator EPS (.eps):
- Select Save or Save As... or Save a Copy...
- In the Format popup menu, select "Illustrator EPS (eps)"
- In the "Version:" popup menu, select "Illustrator 8 EPS" and save the file.
Now you can import the file into Design 3D SE.
There are some advantages to using postscript over the newer PDF (which SE does not support). The biggest reason to use Poscript over PDF is to retain control handles on straight edges: they are removed in the PDF, but are important for some modeling tasks.