how to download IOS that is said to be not compatible with this computer

How old is your Mac? Are you sure if your machine is not compatible with this OS X it can be used on your Mac?

<OS X Mountain Lion system requirements - Apple Support>

To install Mountain Lion, you need one of these Macs:

• iMac (Mid 2007 or newer)
• MacBook (Late 2008 Aluminum, or Early 2009 or newer)
• MacBook Pro (Mid/Late 2007 or newer)
• MacBook Air (Late 2008 or newer)
• Mac mini (Early 2009 or newer)
• Mac Pro (Early 2008 or newer)
• Xserve (Early 2009)

Your Mac needs:

• OS X v10.6.8 or OS X Lion already installed
• 2 GB or more of memory
• 8 GB or more of available space

If your system does not meet the requirements, then the App Store is not going to download the "Install OS X Mountain Lion" installer.

If you have another newer Mac, or a friend with a newer Mac, you could use your Apple ID on that system to complete the download, then "Quit" the installer when it starts, and then transfer the Applications -> "Install OS X Mountain Lion" app to your external disk for the future.

Plus, once you complete the download, it should remain in your App Store -> Purchases tab, so you can download it again in the future on a qualified Mac.