There are two ways to get an object instance from the database:
If you have or can construct a valid ObjectId, then you can get an instance using the getObjectById method of the PersistenceManager as desribed in section "Get Object By Id".
You may also get JDO instances using the JDO Query interface as described in detail chapter "Queries". You get an instance of the Query interface from the PersistenceManager and may specify different search criteria.