We do not sell CS 5.5 anymore. It is an old version with limited support. You should go for CC version, in which you will get the most advanced features and complete support as well. If you specifically need CS 5.5, you may need to check any 3rd party re seller, also check the system requirements for older version apps as latest Operating Systems are not compatible. Please see : System requirements for Adobe InDesign CC for Mac and Windows OS
We need to add a seat license. We already have other workstations with 5.5. How can we just get the license for one more?
Shop around and do it carefully. Chances are anything you’ll find at this point is counterfeit.