If like me you get a 0x800F0906 (Windows 8 can’t connect to Windows update to pull down the files) error while trying to install the .Net Framework 3.5 on Windows 8 RC you can use the files that are included on the Windows 8 ISO / DVD. These files are located in the “\sources\sxs”
You need to use the DISM (Deployment Image Servicing and Management Tool) which will allow you to change the source for the install files.
From an elevated command prompt window run this command (assuming D:\ is where you have the files)
Dism /online /enable-feature /featurename:NetFX3 /All /Source:D:\ /LimitAccess
The install will take a couple of minutes to complete and then you will see a confirmation it has been successfully installed:
If you only have an ISO then you can use a tool such as 7Zip to crack it open and copy the files to your local machine. Or you can use the new ISO mounting feature in Windows 8 itself!!