Put the downloaded "struts-config_1_3.dtd"
file in WEB-INF folder.
Also from struts-config.xml in DOCTYPE tag replace the following
<!DOCTYPE struts-config PUBLIC
"-//Apache Software Foundation//DTD Struts Configuration 1.3//EN" "http://struts.apache.org/dtds/struts-config_1_3.dtd">
with
<!DOCTYPE struts-config PUBLIC
"-//Apache Software Foundation//DTD Struts Configuration 1.3//EN" "struts-config_1_3.dtd">