A non-govermental employer sponsors a money purchase plan and requires employees to contribute 6% of their salary to the plan through salary reduction. There is no written salary reduction agreement. The plan document refers to this as an "additional employer contribution", which is 100% vested at all times. Still, I believe it should be treated as an employee contribution subject to FICA tax and income tax withholding. Does anyone know if that is the case?

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Did this arrangement receive a determination letter from the IRS? Under IRC 401k only a profit sharing plan (or a pre ERISA money purchase plan) can permit employee salary reducton pursuant to an agreement by the employee to defer salary. If there is no salary reduction agreement how is the employer obligated to contribute the employee contributions to the plan?

It's a mandatory employee contribution -- these are permitted under qualified plans, including money purchase plans. The IRC 402 regs suggest that salary reduction agreements are taxable employee contributions unless specifically excepted -- eg per the IRC 401(k) qualified CODA rules. See Reg. 1.402(a)-1(d)(1). But I have not found any specific authority addressing mandatory employee contributions in non-governmental money purchase plans.

Mandatory employee contributions are permitted on an after tax basis to a money purchase plan as a condition of employment. See rev. rul 69-277.But an after tax contributon is not made by salary reduction as you indicated initially. The only difference between a salary reduction contribution and an after tax contribution is that the salary reduction contribution is not subject to federal income tax. Both types of contributions are subject to FICA tax.