If your lexer and parser grammar rules are contained in a single file
then you can debug it (for example, AST generation) in ANTLRWorks
without using remote debugging or writing any Java code. To do this,

start ANTLRWorks
open the combined grammar file
select Debug... from the Debugger menu
enter the input to be tested or select a file to be used as input

However, if your lexer and parser grammar rules are in separate files
then you must use remote debugging. To do this,

generate classes for them using org.antlr.Tool and the -debug option
write a main class that uses them to parse some input
compile your code and the generated code
run your application which will pause waiting for the debugger
start ANTLRWorks
open the parser grammar file
select Debug Remote... from the Debugger menu
press the Connect button

Do not pass null to the constructor of your Parser or TreeParser class
and attempt to set the stream they read from later
(using setTokenStream and setTreeNodeStream).
This will result in a NullPointerException in DebugTokenStream.