Congratulations on your new arrival, I'm a new dad myself and it's the best feeling ever.
Your isofix points are located in between the rear seat base and the back - if you slide your fingers in where the base and back meet and move your hand left to right across the seat you'll find two metal "D" shaped loops about 8-10" apart. These are your fixing points. The arms on your car seat base will give you an idea of how far apart and where to find the points.
ISOFIX bases aren't always easy to fit though depending on the base itself. Try and ensure the arms which go into the seat are extended out as far as it will go first. Sometimes it does require a bit of a shove to go in. Fitted ISOFIX bases to VW Golf MK5 / Pug 306 / Merc C class / BMW F30. Easiest was the Golf 5 but the rest were a PITA. Haven't yet fixed one into my 407 though!
But to answer your question, yes, 407 has ISOFIX in both rear seats, heck if a '99 Pug 306 has 'em, a 407 definitely has them!