I noticed that the specific sublayer of the map service you mentioned has Multipoint features, which I dont think is supported. The other layers which are Points added OK for me - e.g. I tried the bus stops layer.
I was not able to add your specific service sublayer, but was able to add the whole service from here:
http://maps.co.palm-beach.fl.us/ArcGIS/rest/services/ags1/MapServer
I did notice that by default your service layers were all turned off, and had to turn on specific layers (which worked just fine in Explorer Online).
I was able to add that specific sublayer successfully to the map viewer. So your workaround for now will be to use the map viewer to author your map with that particular sublayer, then take it across the finish line with Explorer Online.
- bern