The test form is only available for
requests from the local machine
HTTP GET and HTTP POST may be enabled by editing the Web.config
file for the vroot where the Web service resides. The following configuration
enables both HTTP GET and HTTP POST:
<configuration>
<system.web>
<webServices>
<protocols>
<add name="HttpGet"/>
<add name="HttpPost"/>
</protocols>
</webServices>
</system.web>
</configuration>
http://support.microsoft.com/kb/819267
Related Terminology : Web Service, HTTP GET, HTTP POST, Invoke