What is the impact of a Medical Marijuana card on employment?

That completely depends on the employer.

The employer has no legal mandate to accommodate the use of Medical Marijuana. He may choose to fire a cardholder once the use of Medical Marijuana is known. He may refuse to hire a cardholder. He need not make any arrangements for a Medical Marijuana user to use their medication at the workplace.

Unlike other disabled people, Medical Marijuana patients have no protection at all with regard to employment.