if you're not using ArcGIS 10, then the symbology from the map service isn't used. You can set a default symbol by using the symbol attribute of the FeatureLayer, or you can use a custom Renderer to set different symbols for different attribute values (similar to Categories, Quantities, etc in ArcMap). Here are some examples. The second one is straight from http://help.arcgis.com/en/webapi/flex/samples/index.html
<esri:FeatureLayer id="featureLayer"
url="http://sampleserver1.arcgisonline.com/ArcGIS/rest/services/Demographics/ESRI_Census_USA/MapServer/0" }">
<esri:symbol>
<esri:SimpleMarkerSymbol color="0xFFFFFF" alpha="0.8" size="10" style="diamond">
<esri:outline>
<esri:SimpleLineSymbol color="0xFF0000" style="dash" />
</esri:outline>
</esri:SimpleMarkerSymbol>
</esri:symbol>
</esri:FeatureLayer>
<esri:FeatureLayer outFields="[FIPS,POP2000]"
url="http://sampleserver1.arcgisonline.com/ArcGIS/rest/services/Demographics/ESRI_Census_USA/MapServer/0">
<esri:renderer>
<esri:ClassBreaksRenderer attribute="POP2000">
<esri:ClassBreakInfo maxValue="61" symbol="{symbol1}"/>
<esri:ClassBreakInfo maxValue="264"
minValue="62"
symbol="{symbol2}"/>
<esri:ClassBreakInfo maxValue="759"
minValue="265"
symbol="{symbol3}"/>
<esri:ClassBreakInfo maxValue="1900"
minValue="760"
symbol="{symbol4}"/>
<esri:ClassBreakInfo minValue="1901" symbol="{symbol5}"/>
</esri:ClassBreaksRenderer>
</esri:renderer>
</esri:FeatureLayer>