When you create a new X10 class in X10DT, a dialogue pops up letting you set superclass, implemented interfaces etc.
Type a name for the supertype. Now select browse. You are expecting it to show various X10 types with the given name that can serve as a supertype.
Instead you are shown various Java types with that name.
While you are fixing this, please also check that the other selections in this window work correctly (e.g. browsing for interfaces gives you X10 interfaces) etc.