You can specify which users have access to each individual webservice in the umbracoSettings.config in the config folder.  You can use any of the users you have setup in Umbraco.

  <!-- configuration for webservices -->
  <!-- webservices are disabled by default. Set enable="True" to enable them -->
  <webservices enabled="True">
    <!-- You must set user-rights for each service. Enter the usernames seperated with comma (,) -->
    <documentServiceUsers>your-username</documentServiceUsers>
    <fileServiceUsers>your-username</fileServiceUsers>
    <stylesheetServiceUsers>your-username</stylesheetServiceUsers>
    <memberServiceUsers>your-username</memberServiceUsers>
    <templateServiceUsers>your-username</templateServiceUsers>
    <!-- type of files (extensions) that are allowed for the file service -->
    <fileServiceFolders>css,xslt,data</fileServiceFolders>
  </webservices>

I replaced your-username with Administrator or admin, however i am not able to authenticate to webservice.

Received following exception.

System.Exception: The user does not authenticate

OK i found the answer by means of reverse engineering the source code of Umbraco...

Password was stored in HASH format in DB and i was sending plain text password through webservice and hence authentication was failing.

is working on a reply...