The UNESCO headquarters is a terribly frustrating place to get to visit.
I was over in Paris a few months ago and just gave up in my efforts to try and see something there. There was no information on any of the UNESCO pages that I could find, all other tourist info suggested you could arrange a guided tour, but there wasn't any contact details.
Most just direct to this page
which has been taken down, but apparently this is the phone number for them 33 (0) 45 68 10 00 if you want to call and see if you can get any further.
The best I could find is this amusingly out-dated 'Virtual tour'
which looks like it was designed a good 15 years ago, which in internet terms put it at the equivalent vintage as Stonehenge.
It looks like there is a decent selection of C20th art works on display, I was really keen on visiting Tadao Ando's 'Meditation space' made using granite from Hiroshima, but alas it was not too be.
Just in case it is of interest you could get in a pre-emptive strike on a likely future WHS by visiting some of the le Corbusier buildings, the best is Villa Savoye
in Poissy (about 40 mins from central Paris) or there are the buildings a little closer to the centre, though these may get culled in the re submission of the proposed WHS
is some info on visiting them.