Thanks for your question.
The Principle Business Activity Codes are listed on pages 39-41 of the Form 1120S instructions: http://www.irs.gov/pub/irs-pdf/i1120s.pdf
I think yours would be 541400
"Specialized Design Services
541400 Specialized Design Services (including interior, industrial, graphic, & fashion design)"
Hope this helps.
Thanks ever so much, I've been driving myself crazy trying to navigate all the information and never being able to locate those six illusive numbers.