All versions of Adobe InDesign CC are compatible with SignAgent Pro™. If you do not own a compatible version of InDesign you will need to purchase Adobe InDesign CC.
UPDATE: InDesign has released the newest version, CC 2017, and current versions of SignAgent Pro™ are completely compatible with this new release.
Compatible versions of Adobe InDesign
* Adobe InDesign CC (ALL)
* Adobe InDesign CS 6. (Unless you already own Adobe InDesign CS6 you should purchase Adobe InDesign CC)
Incompatible versions of Adobe InDesign
Adobe InDesign CS5 & 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