There is a bug in studio beta 6.2.993, when inside a freshly installed studio it is impossible to start a debug session using the internal debugger, whatever the parametres in project.
Once an AVRISP mkII has been inserted and selected a s tool, it is possible to come back to simulator ( even if the mkII is not present any more) and the debug session can be started.
With the initial installation no debug tools is shown in the tool bar. After mkII insertion and simulator selection, the last one is shown on the tool bar.
Regards,
Pierre