Since you have the Dataservice is implemented, give that as endpoint url to your proxy which can be created in wso2esb. When you send request to your proxy,in the outsequence, you will receive the response of your dataservice. Just do a log with "level=full" you will see the full response. Use the property mediator and do an xpath to pick the value which you needed. Sample conf:
<proxy name="StockQuoteProxy">
<target>
<endpoint>
<address uri="DS endpoint"/>
</endpoint>
<outSequence>
<log level="full">
<property name="email" expression="xpath from the email attribute in the rseponse"/>
</log>
<send/>
</outSequence>
</target>
</proxy>
Here is esb sample guide on how to create proxies;
http://docs.wso2.org/wiki/display/ESB460/Proxy+Service+Samples