When you enter your 1099-MISC for your board meeting stipend and click Continue, the screen Does one of these uncommon situations apply? should come up. Choose the second option [Name] was on a board of directors; this was [Name's] fee.
If that doesn't work to populate Box B of the Schedule C, then you may choose the option "999999 - Not Otherwise Classified." The IRS business codes are not very detailed.
