Dash rattles are a common problem with Meganes sadly
The top part of the dash where the radio display is comes out quite easily; take a firm hold of the cover and lift up and towards the windscreen, the bottom edge should lift out enabling you to get your fingers underneath which helps you even more.
The instument binnacle lifts forwards and up (be careful as the clips can be fragile). Put your fingers at the back and gently prize it up whilst pulling it forwards.
Never had the vents out of mine but took the radio out and plastic cubby hole cover out several times, this has two plastic clips that are easily prized down with your thumbs and it then just pops out with some gentle persuasion.
I found thin foam window draft excluder (from B&Q) to be very good a stopping the plactic to plastic contact that often causes creaks and rattles.
Your pollen filter is located deep down at the back of the dash, look at the back of the centre console from the drivers footwell and you should see a small tab sticking out, this is the filter.
Good luck hunting them rattles!
