As answered above, your issue isn't actually with Objectify, but rather how the datastore works. You will find it very difficult to work effectively using objectify without understanding the basics of the datastore. You can read the general docs here and specifically about querying here.
The short answer is, that the datastore is only able to match entities to queries if the fields you are querying on are indexed for that entity. Objectify gives you an annotation to achieve this with, consistently across all entities of a given kind.
The reason you must specify each field to index is that indexes have a cost associated with them when you write entities, so having fine grained control is important.