This study will evaluate whether the addition of sitagliptin reduces hemoglobin A1C (A1C) more than the addition of placebo for participants with type 2 diabetes mellitus (T2DM) on a steady dose of acarbose. The primary hypothesis is that the addition of sitagliptin 100 mg once daily (q.d.) reduces A1C more than the addition of placebo in participants with T2DM with inadequate glycemic control on acarbose monotherapy.