Whatever "filesel" is, it doesn't look like you've created it to be a "GtkFileSelection" widget (ie. some "GtkWidget *" created with a call to gtk_file_selection_new()).
Please compare your code with the sample code here:
http://developer.gimp.org/api/2.0/gt...Selection.html
If you're not using the code generator (i.e. if you don't have any non-glade GTK+ code to look at), then it might be a glade problem. In that case, you might want to consider just doing the GtkFileSelection manually (instead of via glade). It's easy (as the example code in the reference above shows).
Good luck .. PSM