Looks like the SSD is a mSATA device that plugs into a PCIe interface on the motherboard. It's there to cache requests to/from the hard drive (similar to a 'hybrid drive')
While there might be a way to format it and assign it a drive letter I would recommend against it. You would be left with a very small SSD drive and slower traditional drive.
Recommend you format and install the OS on to the standard hard drive. After the OS is installed you will probably need to install additional software to enable using the SSD for caching. Contact ASUS support for this.
You can also review this article for more information about SSD caching using Intel chipsets.