I solved it, after some more searching in Google and in the forum.
I had simply to use the IP2010 forms (and not IP2007) and set the query field of the data connection (see i.e. http://www.wonderlaura.com/Lists/Posts/Post.aspx?ID=129)
Did not try before to use IP2010 forms as I was not sure if it worked with my SP 2007
see you ...