ATL is for model-to-model transformation.
If you want to generate code out of models (obtained thanks to such model-to-model transformations for instance), you have to use model-to-text technologies such as Acceleo.
If you target the generation of Java code, the MoDisco project already provides you the Java metamodel and Java generator for free.
You then just have to build your ATL transformation from XXX-to-Java (with XXX being your source metamodel, such as UML for instance).
As I told you in my previous post, in your specific case where you start from an UML model, you have to build an UML-to-Java transformation in ATL and then directly use the provided MoDisco Java generator.
Discoverers are for getting models out of artifacts which are not models (e.g. source code, configuration files, data, etc), that is not your case from what I understood.
Everything is pretty well explained in the MoDisco documentation.
The generate Java Source discoverer provided in MoDisco 0.10.1 is not available from the menu in the IDE, because it still depends on the org.eclipse.gmt.modisco API. But that is deprecated as it was moved to org.eclipse.modisco. I wrote my own discoverer to accomplish what the provided one does.
The Java Source code generator has never been directly available from the IDE as the discoverers are.
It is an Acceleo plugin project that needs to be either added to your (under development) plugin's dependencies or checked out onto your (development) workspace.
Then, you can launch it (from your Java model) respectively using either Java code or an Acceleo launch configuration.
Cf. the "Java generation" section of the MoDisco documentation.