Manage devices
If your device does not show up under manage devices -- how do you get it to show up.
I will assume that you mean that a device does not show up as being associated with your Apple ID.
Manage your associated devices in iTunes - Apple Support
Try downloading any past purchase to the device that does not show up. That should associate the device with your ID and should make it appear in associated devices. Make sure that you are signed into the Apple ID that you want to associate with the device with and go to the purchased tab of the App Store, iTunes or iBooks and download anything that you have already purchased but that is not already on the device.