You can debug a script task only when the below condition are satisfied.
1.You can debug the Script task when you run the package in 64-bit mode.
2.You must execute the package to debug into your Script task. If you execute only the individual task, breakpoints in the Script task code are ignored.
3.You cannot debug a Script task when you run the Script task as part of a child package that is run from an Execute Package task. Breakpoints that you set in the Script task in the child package are disregarded in these circumstances. You can debug the
child package normally by running it separately.
4.When you debug a package that contains multiple Script tasks, the debugger debugs one Script task. The system can debug another Script task if the debugger completes, as in the case of a Foreach Loop or For Loop container.
Refer http://technet.microsoft.com/en-us/library/ms135952.aspx
Regards, RSingh