You may find it useful to take a look at the notes in the "getting started" section of the MPXJ web site. To summarise briefly, data from Microsoft Project, and other project planning tools, typically consists of a top level project, tasks, resources, and assignments (which link tasks and resources together).
This is pretty much how MPXJ represents the data read from a project plan. The attributes of each of these objects can be set or retrieved using the relevant set and get methods on each object. So for example, the Task object in MPXJ will expose setStart() and getStart() methods to allow you to work with the task start date. The method names follow the names used for the attributes in Microsoft Project so hopefully you will find it stratightforward to locate the attributes you need. You may also find the API documentation helpful in this respect too.