Classipress is also a wordpress theme and to implement it with your existing wordpress site is to have a separate install of it either in a folder or subdomain. Then you may just add a navigation menu on your existing site that points to the classipress install.
Please help our moderating team work more efficiently by not sending us support questions via PM. You can read more about how AppThemes support works here. However, you can send a PM to follow up and remind me if I missed your support request/thread.
