On the "API Access" page for a project in the Google APIs Console you can find this value under "Key for server apps", in the "API key" field.

If you don't already have a "Key for server apps" in your API project, then click the "Create new Server key" button, then click "Create" on the dialog that appears. Ensure that you don't copy the API key from the "Key for browser apps" section, if it exists.