I tried to enter 25% for SEP and Turbo H&B won't allow any contributions. I haven't entered any other retirement data, yet, so I don't know why it won't allow a SEP contribution.
You should be able to deduct the SEP IRA in this instance, but the maximum contribution is approximately 18.59% of your net self-employment income.
Circling around on this question - If I'm the only person under sole proprietorship, do I file as employer or employee?
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%).