Yeah, the trial version never showed the PS3 connected under the devices tab either...
Make sure, on your PS3, that the "media server connection" option is turned on (settings --> network --> media server connection). You may have to restart your PS3 and/or restart the MediaLink sharing for them to find each other.
It worked for me after I did this, but oddly enough the PS3 never showed up on that list until I bought the license for MediaLink.