Mapping Center Answer:

This isn't possible in ArcGIS Explorer directly as it's designed as a viewer for maps, layers and services that already exist. You can view attributes in a pop-up by configuring the Layer Properties to use the value of a field as the pop-up content.

You can modify symbolization based on attributes in ArcMap and then save the layer as a layer file (right click the layer > Save as Layer File). You can add the .lyr file to ArcGIS Explorer using Add Content > ArcGIS Layers.