WEKA Random Forest J48 Attribute Importance
If you must use WEKA, I would suggest looking in the Attribute Selection panel. In this panel, you can rank attributes by information gain, as well as look at which subsets of attributes perform best. To my knowledge, you can't get this information directly from the classifier in the WEKA Explorer, but it is quite easy to get in other frameworks such as scikit-learn: see this example in the docs.