We need to add Baseline Usage field to an ad hoc view, but even though it' s listed as an attribute of the Project object, it is not available on the list of fields that are available for ad hoc views of the Project Management domain. How to make this field available for the view?

Release:
Component: ODJSP

Currently the 'Baseline Usage' attribute on Project Object is not Data warehouse enabled by default.
In the Project domain of within Advanced Reporting, if you drill down as follows then you can get Total Baseline Hours. 

Project => Summary Totals =>Project => Total Hours => Total Baseline Hours 
or 
Project => Summary Totals => Task => Task Hours => Task Total Baseline Hours 
or 
Project => Summary Totals => Assignment => Assignment Total Baseline Hours