Directly from the IRS web site:
The Fix
As in the case of an erroneous exclusion of an employee from the plan, the remedy requires the employer to make a corrective contribution of 50% of the missed deferral (adjusted for earnings) on behalf of the affected employee. The employee is fully vested in those contributions, which are subject to the same withdrawal restrictions that apply to elective deferrals. However, unlike in the case of mistaken exclusions where the missed deferral amount is estimated based on the ADP for the employee category (e.g., NHCE), in both illustrative examples, the employees’ election deferral amount is known. Thus, the missed deferral and the corresponding corrective contribution (50% of the missed deferral) are based on the participant’s actual election instead of the ADP (i.e., 5% or 3%, respectively) of the NHCEs.
Here is a link that might help...
http://www.irs.gov/retirement/article/0,,id=175716,00.htmlPAL