Connecting to users local machine from my WebApp

Question

I have a Web App that i have developed I am looking to develop a plugin for this app so that i can pull information from the users accounts package such as sage or quickbooks(offline version). Ideally I would like to do this without them having to have a static ip etc.

Unfortunately this is way over my head I currently have no idea on how to do this.

Please can some one guide me in the right direction I will probably use json data between the webapp and the client service that I will make and im fine with that part its just the initial connection that I am confused about.

Any help would be really really appreciated I hope this makes sense.

Thanks

Ash