While playing around with playframework yabe-siena-gae, I noticed a datastore file created for the sample data.
Does any tool exist or is there some way to view the data inside the datastore file created by siena?
If you are using the out of the box Play Server with the Google App Engine module, then you can't unfortunately.
The post does suggest that if you are deploying to a local servlet container (i.e. Tomcat/JBoss, but not the Play Netty server) then you may be able to use the standard data viewer URL that comes with GAE
localhost:8080/_ah/admin/
Alternatively, deploy a test version to a GAE account, and use the data viewer from within the google web consoles for managing your apps.