Some rather convoluted fixes were suggested by Microsoft in TechNet articles, but the solution I used was to simply mount the Server2012 ISO file within the VM, locate the Sources\SxS directory, and give the wizard this path as the ‘alternate source’. Didn’t even need a reboot!
[EDIT – 09/02/16] – The same issue has been seen on Win 8/10 but the solution is a little different (albeit the above may work, an alternate method without the need for disks etc is available).
If you are doing this on a physical server and have the OS DVD to hand you can simply point it directly at that…
[EDIT – 23/05/16] If, as I’ve just encountered, you don’t get the option to select an alternate source, then you can run the following command from an elevated command prompt:
Dism /online /enable-feature /featurename:NetFX3 /All /Source:D:\sources\sxs /LimitAccess
Replace the drive letter with that of your drive/mount point of course.