In order to debug a tree grammar the input must be an AST.
Presently the only way to gain access to AST input is through creating a parser that provides "remote debugging".
See How do I set up remote debugging anyway?.
It is necessary to first build an AST that will serve as input to the tree grammar parser.
This parser (sample.g) will not be a debug parser and must not be generated with the -debug option. Only the tree grammar (sampleWalker.g) source should be generated with the -debug option

sample.g

grammar sample;
example : 'SOME TEXT TO PARSE';

sampleWalker.g

tree grammar sampleWalker;
example : 'SOME TEXT TO PARSE';

remoteDebugDriver.java

public class remoteAstDebugging
{
    public static void main(String[] args) throws Exception {

      CommonTokenStream tokens = new CommonTokenStream();
      {
         ANTLRInputStream input = new ANTLRInputStream( System.in );
         sampleLexer lexer = new sampleLexer(input);
         tokens.setTokenSource(lexer);
      }

      CommonTreeNodeStream nodes;
      {
         sampleParser parser = new sampleParser(tokens);
         sampleParser.example_return example = parser.example();

         CommonTree tree = (CommonTree)example.getTree();
         nodes = new CommonTreeNodeStream(tree);
      }

      {
         sampleWalker walker = new sampleWalker(nodes);

         // remove these lines if not using templates
         FileReader groupFileReader = new FileReader("cpp.stg");
         StringTemplateGroup templates = new StringTemplateGroup( groupFileReader );
         groupFileReader.close();
         walker.setTemplateLib(templates);

         sampleWalker.example_return example = walker.example();
         System.out.println(example.toString());
      }
      System.exit(0);

    }
}

This program blocks when

sampleWalker.example_return example = walker.example();

is executed, waiting for a remote process to connect. From within ANTLRWorks open sampleWalker.g and select "Debugger -> Debug Remote", using the default host and port, localhost:49153. This allows ANTLRWorks to take control of the remote parsing operation.