While attempting to install .NET Framework 3.5 from Add Roles and Features fails with the error:  "The source files could not be found".  Attempting to install from the .NET 3.5 Framework install fails pointing you back to Add Roles and Features.

You have two options:

1.  From an elevated Command Prompt, run the following:

dism.exe /online /enable-feature /all /featurename:NetFX3 /Source:D:\sources\sxs /LimitAccess

2.  From within Add Roles and Features, after you select the feature, down at the bottom, select "Specify an alternate source path", enter the following:

D:\sources\sxs

NOTE:  The above assumes the DVD drive is D:, if not, change the path drive letter to reflect the appropriate drive letter.