Are these the seals at the bottom of the glass - if so I think they just clip in place and take only about 15 minutes to replace 2 of them. You could try pushing them back in place but if this does not work maybe they need replacing. If the car still has a warranty I would think they should be covered. If it is a Citroen warranty you could try another Citroen dealer if this one does not want to help. If it is the seals around the door that the glass seals against when closed, then I think these also just clip in place.
The seals around the door aperture are also clipped to the bodywork but if coming loose then maybe these need replacing if the clips built in to them have opened up. It might be they have been removed at some point for a respray, has the car had any bodywork done to it ? These seals are not cheap from a dealer if you need to pay for them so another source could save some money. If the clips have opened up you could try nipping them together first.
I would check the door drain holes are not blocked and that the carpets are not wet which could be causing the misting up.
There is a common fault on the C4 Picasso which you need to check. The tailgate spoiler can crack at the top corners needing a new tailgate to sort it. Have a look at this topic on a different forum http://c4owners.org/...ic.php?209211.0