I am using SP2013. When I create a workflow in SP designer I do not see the 2013 platform. I need to install it by the Workflow manager. Do I need to install the workflow manager on the WFE or APP server?
According to this slide, you can install Workflow manager on WFE server but you still need to install workflow client and go through procedure of configuring.
You will need to install Workflow manager client on all WFEs regardless of as if Workflow manager is installed and configured on App or WFE server.
This MSDN article will help you understand the concept, http://technet.microsoft.com/en-us/library/jj658588(v=office.15).aspx