The brakes on our M&P stroller (Sport 03 version) have always worked well, but can be a bit awkward to engage. They work by a metal pin being pushed (by the footplate) into the plastic gear-tooth pattern on each wheel. To engage the brake, the stroller needs to be rocked back and forth a bit to get both pins to push into the gaps between the gear teeth.
If you're getting no braking effect whatsoever, you either aren't pushing the footplate hard enough whilst rocking the stroller, or the teeth in both wheels have all broken off (very unlikely).
First thing to do is: remove the wheels! There's a plastic tab behind each wheel; lift the tab whilst pulling the wheel off.
Second thing: check that both wheels have their gear teeth present. If not, you'll need new wheels. Time for a trip to the Mamas & Papas store! Try not to be talked into buying a whole new stroller though...
Check that the metal pin inside each wheel well (on the stroller itself) moves freely when you push the footplate. If it doesn't, try a bit of WD-40 on the pin to free it up. Note that one sticking pin will stop both of them moving, so you'll have to spray both of them to loosen up the sticking one. If this doesn't help, some other part of the footplate mechanism may be seized up. It's a part of the stroller that can catch a lot of muck and dirt, so you may just need to give the area a wash and then spray any moving parts (hinges etc) with oil.
When you're done, push each wheel back on. You should hear a loud 'click' when the wheel has locked in place.
On a general note, I would strongly recommend that this stroller has working brakes. The stroller moves extremely efficiently on its air-filled tyres, and can easily pick up a fair speed if left unattended.
Final warning: I'm not an expert on strollers! Use your common sense. If you aren't sure what you're doing, just accept defeat and take your stroller along to a dealer.