Open the AST View by selecting Window->Show View->Other and expanding the Antlr Studio node.
When you are debugging a grammar file, the AST View shows the current AST which is being formed, for the current rule only, while if you are debugging a tree parser the AST view shows you the tree transformation taking place for the current rule.
Clicking on the 'Show Parse tree' button shows the parse tree that has been formed.
When debugging a tree parser, clicking the 'Show AST being passed' button shows the AST that is being passed to the current rule.
