Tutorials for Weka, imbalanced data, ROC Curves and AUC?

I am working on a classification project that has imbalanced data, (90-10%) and I am working on the classification rate. I understand the basics of how to use Weka explorer.

Since this data is imbalanced I need to look at the ROC curves and the AUC to determine if I am actually improving my classification rate (and not just labeling everything with the 90% label).

Do you know of any good tutorials that deal with this in Weka? If not, what about in R (though I have little experience with this).

Thanks