Vikash Kumar Singh (5/14/2013)
What tool you are using to design packages? SSIS 2008/2008R2/2012.As per my experience you should always create environmental variables as System Variables.
First point to note is that they're called 'environment' variables, not environmental - these won't help us with climate change.
The link provided by Koen provides all the details - look for the parts which refer to 'indirect' configuration.
If you haven't even tried to resolve your issue, please don't expect the hard-working volunteers here to waste their time providing links to answers which you could easily have found yourself.