You should be able to open the door manually. Have you taken the key out of the fob? If you find it difficult to turn the key use a pair of plyers to grip key but do not undually force.

 

 

Apparently there is a slot in the key fob for the "T" piece of the manual key blade to fit in, it is very stiff to turn it enough without pliers or using this slot.

I've recently got my Kuga and wanted to test how to get in and start the car if my key fob went flat so I removed the fob battery whilst being at home and having access to the spare fob if needed! When I used the manual key to unlock it set the car alarm off after a short delay - had to then place fob in the scanning area in the centre console (near the cup holder) and start ignition for alarm to turn off and yes, the car started just as it should.

The alarm has it's own back up battery - you may find your alarm goes off if you unlock using the manual key even if the main car battery is flat.