You do not enter it in the Schedule C interview as Pension Plan Contributions. I don't have home and business, but there should be a section in the business side for Business Deductions and Credits. Enter it there. Since this is a SEP, it is contribution as an employer, but that shouldn't matter. As you work through the interview, you skip through the section on Individual 401k plans to get to the SEP contributions, and you enter the amount or hit maximize if you want to determine the maximum you can contribute (the 18.59%).
