The Macs which came with ML are licensed to be used on those Macs only. In order to install ML on Macs which came with older versions of OS X, you must purchase ML in the App Store from one of those Macs. Once purchased, ML can be installed on all older Macs you own. Do not use the App Store version of ML on those Macs which came with ML.
This is when you have purchased the ML license.
Go to the mac where you want to install ML and sign in the app store there using the same apple id that prchased the ML license.. Go to purchases and you should be able to download it.
EDIT: try holding down the Option key whie clicking on the Purchases tab