KB ID 0000924 Dtd 07/13/14
I needed to add .Net 3.5 to a Windows Server for an application install, but when I added it as a 'feature' it failed.
Installation¬†of¬†one¬†or¬†more¬†roles,¬†role¬†services, or features failed. The source files cannot be found. Try installing the roles, role services, or features again in a new Add Roles and Features Wizard session, and on the Confirmation page of the wizard, click "Specify an alternate source path" to specify a valid location of the source files that are required for the installation. The location must be accessible by the computer account of the destination server.
1. Open a command Window (Run as Administrator)
2. Insert the Server 2012 Install DVD >Execute the following command;
Dism /online /enable-feature /featurename:NetFx3 /All /Source:D:sourcessxs /LimitAccess Where D: is the DVD Drive Letter