With the closure of the NOAA web service of METAR data (see this announcement) the standard METAR URL used in OpenRadar (http://weather.noaa.gov/pub/data/observations/metar/stations/) doesn't work anymore. After a search I found a proper replacement URL
http://tgftp.nws.noaa.gov/data/observations/metar/stations/
Fill this in on the Settings tab and your METAR will work again.