I recently installed Dreamweaver CS6 alongside Photoshop CS5 already on my system. They both come with their respective Extension Managers, but when I download an extension for Photoshop and click to install from Chrome it opens the CS6 extension manager, which my copy of Photoshop isn't in. Is there a way to put Photoshop CS5 into the CS6 extension manager purely for convenience's sake?
No you must keep your CS5 extension manager and change you default application for installing apps in your browser. It would be better to save the files to your Desktop and then open ExMan CS5 and choose 'Install', then locate the ZXP from the Desktop.