The second suggestion is not possible. You are looking for all calls to add that originate from the field mList. However, AspectJ works on instances at runtime and what you want to do is not defined at runtime.
Foo f = new Foo();
List newList = f.mList();
newList.add(null); // should the advice be triggered here?
This is just not something that AspectJ does.
However, your first suggestion seems like it should work. There are a few reasons why it may not. For example, is the "foo" package binary and is it on your Aspect/In path?