By default, for security reasons, the web service test form is only available when looking at the web service information page on the machine that the API is installed on. If you try to browse to a test form from another machine, you get a message "The test form is only available for requests from the local machine."
For development purposes, you can enable access to the test form on other machines by editing the web.config file. In that file, find the <system.web> element, and add this directly under it (so that, XML-wise, it is still within the system.web element):
<webServices>
<protocols>
<add name="HttpPost" />
<add name="HttpGet" />
</protocols>
</webServices>