The first step in installing Ex Nihilo is to set up a profile for 1.6.4 on the Minecraft application. To do this, open minecraft and select Edit Profile. Near the bottom middle it displays a version. Select 1.6.4 and then save profile.

The next step is to download and install Minecraft Forge . The easiest way to install it to download the 1.6.4 installer and let it do it's magic. Once Forge is installed, we can move onto the penultimate step: Ex Nihilo.

To install Ex Nihilo first download it from the Minecraft Forums . Then, in your .minecraft folder (%appdata%/roaming/.minecraft), create a folder labeled 'mods' and simply drag the Ex Nihilo mod into the folder. 

All that's left is to fire up Minecraft and play some Skyblock!