To upload data to the datastore I use this java code :
DatastoreService ds = DatastoreServiceFactory.getDatastoreService();
Entity entity = new Entity("mydetail");
entity.setProperty("entry", "entry");
ds.put(entity);
For uploading form based data is this the correct method of uploading data, ie using similar code above or is there another API I should be using ?
Yes, this the direct API to the AppEngine Datastore.
You can also use JDO interface which allows for directly storing a Java object without dealing with the Datastore API:
There is also the JPA interface. Both of the interfaces are described on the App Engine website.
The Objectify interface is very easy and for many situations easier. It is not part of the official SDK.
You can use whichever makes more sense for you application.