Follow the article: http://jaliyaudagedara.blogspot.sg/2015/10/using-aspnet-web-api-as-data-source-for.html
Need to configure the web api service to always return XML data. When configuring the data source, add ?type=xml at the end of the URL.
Note: for SSRS/Report Builder to retrieve the fields, don’t use blank query string, use the following:
<Query>
</Query>