There are two ways to do this:
- Book through the Amex website. They have their own travel agency and allow you to pay with MR miles. Your fare class will be an miles-earning class.
- Convert your MR miles to flying blue and redeem for an award ticket. There needs to be availability, and like Irishguy28 already mentioned: Fees are probably more than the cheapest promo ticket in economy. Redeeming in business makes a little more sense, but will still require a boatload of miles for essentially a 1 hr flight. (=not worth it)
To book a flying blue award, enter through KLM.com or airfrance.com and set the location to CZ, and language to English. Then you can choose PRG as departure.