I haven't seen a schema for the ServiceDesk database. I just had to figure it out by looking at the tables and the data in them. The main tables you need to use when reporting are ServiceDeskIncidentManagement, ReportProcess, Task, and User. Once you figure out the relationship between incidents, processes, and tasks it isn't too hard.
One way to become more familiar with the schema is to look at the SQL for an existing report, by editing it and clicking on the Options tab, and clicking Display SQL.