arcgis viewer for flex 2.1 was not support wms but in 3.0 you can use wms. more info
Possible layer types are as follows:
tiled—Cached ArcGIS map service (9.3 or later).
dynamic—Non-cached ArcGIS map service (9.3 or later).
feature—Single layer in an ArcGIS map service or feature service (9.3 or later).
bing—Microsoft Bing layer (requires a Bing Maps key).
image—ArcGIS Server Image Service (9.3 or later).
arcims—ArcIMS Service (9.3 or later if using a Web Mercator projection).
osm—Basemap from OpenStreetMap.
wms—WMS service. [Added in version 2.2]
kml—KML feed.[Added in version 2.4]
for adding wms in your config file you should glance at "WMS as a service in Flex Viewer" here.
i hope it helps you...