All versions of Adobe InDesign CC up to 2018 are compatible with SignAgent Pro. If you do not own a compatible version of InDesign you will need to purchase Adobe InDesign CC.
UPDATE: Note that newer versions of InDesign (2019 and 2020) are not compatible with SignAgent Pro scripts.
Compatible versions of Adobe InDesign
* Adobe InDesign CC (up to 2018)
InDesign 2018 is available for an easy and free downgrade for anyone who has an active Creative Cloud subscription. You can see details on that here: Install Previous Version of CC Application
Incompatible versions of Adobe InDesign
Adobe InDesign CS6 & Earlier
All other version of Adobe InDesign are incompatible with SignAgent Pro™. You may be able to open the file. However, there is no guarantee that the scripts will function correctly.
You can learn more about InDesign here: http://www.adobe.com/products/indesign.html