There is an existing thread (30 odd pages ) for the Gallery plugin here...
http://forums.b2evolution.net/viewtopic.php?t=7880
Maybe your better off sticking with that thread if that's the Gallery your referring to.

Also bold resized quotes are not required.

hong0,

Sorry for the late reply. The reason you are getting that error is because the gallery2 plugin can't find the path to your gallery2 installation. Gallery2 is a separate program which needs to be installed separately. See http://gallery.menalto.com/

If you are just looking for a easier way to manage photos for your blog then maybe the gallery plugin that John mentioned would be better for you.

Ian