Since you are under age 50, your maximum elective deferral is $18k. For a contribution totaling 24k you would enter $18k as your elective deferral and $6k as your employer match/profit sharing contribution. These entries are to be entered separately in the corresponding boxes on the page for Individual and Roth 401(k) plans under Individual 401(k), without marking the Maximize box.
A Single Member LLC that has not elected to be taxed as a corporation is a disregarded entity and taxed as a sole proprietor reporting on Schedule C. As self-employed, you are your own employer; you make your elective deferral as employee and the employer match/profit sharing contribution as employer.
You can view your Form 1040 by clicking Tax Tools > Tools -> View Tax Summary, then Preview my 1040 (at the left).
