Here is an example of the syntax for DWITHIN
http://demo.opengeo.org/geoserver/wfs?request=GetFeature&version=1.0.0&service=WFS&typename=topp:states&outputformat=json&cql_filter=DWITHIN(the_geom,Point(-88 38),0.1,kilometers)
Note that DWITHIN takes three parameters: reference geometry, distance, and distance unit. However, GeoServer does not really support units so that it could convert kilometers to degrees. In this example the topp:states is having degrees as units and degrees is also used for building the buffer even I wrote kilometers as units. Units does not really do anything but it is a compulsory parameter for the DWITHIN and degrees does not belong to the list of accepted units. I consider this as a bug in GeoServer.
Acceptable units are one of: "feet", "meters", "statute miles", "nautical miles", "kilometers".