Dear Hasan,
The positive/negative values are well explained in the Source estimation tutorial:
http://neuroimage.usc.edu/brainstorm/Tutorials/SourceEstimation#Sign_of_constrained_maps
People in EEG source modeling tend to use unconstrained models, as illustrated in the EEG/Epilespy tutorial, because it produces smoother maps. EEG does not have the spatial resolution we can expect from MEG, the information we collect at the surface of the head is a lot more smeared, so there is no hope to localize precisely the activity within a sulcus.
http://neuroimage.usc.edu/brainstorm/Tutorials/Epilepsy
Please read carefully all the introduction tutorials and get back to us if you have more questions.
Thanks,
Francois
(see your other post: http://neuroimage.usc.edu/forums/showthread.php?2687-Export-channels-from-EEG-Data)