When creating a new data mapping activity or editing an existing one, the XSL generated within the XSL tab may become blank.
A mapping rule, local variable, or other entity within the data mapper is invalid prohibiting the XSL from being generated properly
1) Enable Java Console Mode
2) Launch the Data Mapper
3) Click on the XSL tab
4) View the error thrown in the Java Console window when clicking on the XSL tab. This should help debug what could be causing the issue.