See documentation on our User Guide:
If you fork this you'll need to use something to set the source-url correctly in your manifest.
Info on how the download works: https://developers.openshift.com/en/get-involved-extend-openshift.html#how-cartridges-are-downloaded
Easy way to work around issues: https://developers.openshift.com/en/get-involved-extend-openshift.html#the-cartridge-reflector
For example, to install the latest version to a new app called "test":
rhc app create http://cartreflect-claytondev.rhcloud.com/github/lightblue-platform/openshift-lightblue-cart -a test