I found the solution ...
To enable my USB headset, I had to disable my motherboard
built-in sound card in my PC BIOS.
When I did so, PRESENTER 6.1 is using the only microphone
avalaible : the USB one ...
But it is strange that PRESENTER 6.1 does not provide the
input card choice ...
Every other sound software I know does (like audacity and
so...)
See you