The Error message is advising you to change your Customer Defaults. To get to Customer Defaults:
Menu > File > Utilites > Customer Defaults...
Scroll down the list on the left side and choose:
Simulation > Post Processor
Then select the Results tab
Set the Additional Results File Extension to DES
Here's a link to the SC 12 help file for Setting up Samcef environment defaults which contains some more information that might be useful depending upon the type of simulations you are performing.