If you’ve purchased a multi-seat license for your Sibelius 7 upgrade, your activation and system IDs will be on the Entitlement Certificate. You will only have one of each ID.
All you need to do is install the latest version of Sibelius* and open it. When Sibelius opens, it will ask you for the Activation and System IDs. Enter the IDs in the appropriate fields. Repeat this process for each computer.
*You can download the trial version of Sibelius from the Sibelius website or install from a disc and then update to the latest version.