As a matter of fact, the feature was removed on v4.2 (ref...):
Typed list editor no longer shows the aggregate function column on fields. The aggregate function column is now hidden. It didn't make sense for all other frameworks but our own and even for our own framework it's not that useful: the aggregate function often returns a different type than the one it is placed on, and there was no way to specify the type (which is undefined at design time, as an aggregate function can return different kinds of types depending on which database it is run). In the future the feature will be redesigned if needed.
I recommend you to use TypedView, or any of the runtime/dynamic ways: Linq, QuerySpec, Low Level API.