You can not install plugins for TV.
After you physically installed the card on your computer, your computer probably will install driver automatically. If not, you should run Hauppauge installation CD or download.
To configure your device in JRiver Media Center, go to Tools > Options > Television in MC. In the lower portion is a list of Devices.
You should see at least two devices in the list (the name may be called something other than HVR 5500), one digital and one analog. Select the digital device and click Configure button on the right. On the dialog window, select a tuner type (DVB-T, DVB-C, etc.) - choose one that you want to use this device for. Click OK.
Now you can scan for channels. Near the top of Television Options, click Scan for Channels. Follow the instructions on the wizard.