You need to define a resource reference in your application and then map that logical resource reference to the physical resource (data source) during deployment.
In your web.xml, add the following configuration (modifying the names and properties as appropriate):
<resource-ref>
<description>Resource reference to my database</description>
<res-ref-name>jdbc/MyDB</res-ref-name>
<res-type>javax.sql.DataSource</res-type>
<res-auth>Container</res-auth>
<res-sharing-scope>Shareable</res-sharing-scope>
</resource-ref>
Then, during application deployment, WAS will prompt you to map this resource reference (jdbc/MyDB) to the data source you created in WAS.
In your code, you can obtain the DataSource similar to how you've shown it in your example; however, the JNDI name you'll use to look it up should actually be the resource reference's name you defined (res-ref-name), rather than the JNDI name of the physical data source. Also, you'll need to prefix the res-ref-name with the application naming context (java:comp/env/).
Context ctx = new InitialContext();
DataSource dataSource = (DataSource) ctx.lookup("java:comp/env/jdbc/MyDB");