I can do it if I connect a monitor to the Oppo and then use the Oppo's menu structure to select the files.
When you do this, are you selecting JRiver UPNP/DLNA server on the Oppo, or have you got a network share on your PC that the Oppo is connecting to and you are therefore browsing your PC's directory structure?
If the latter, then this is simply file sharing across the network. You will not be able to push files to the Oppo from the PC in this mode, the Oppo is effectively using the PC as an external hard disk.
If the former, then you have already successfully set up JRiver as a DLNA server and the Oppo is behaving as a DLNA player, i.e. allowing you to browse the server, select items, and play them. However, this does not automatically mean that it is possible to do it in reverse, i.e.
push files from JRiver to the Oppo. As I mentioned initially, the Oppo needs to be a DLNA Renderer for this to happen. Windows calls this facility "Play To".