You have incorrect conditions in where clause, that eliminates value which you want to see. Try this, and correct conditions in where clause to your needs:

SELECT *, dbo.tPeriod.PeriodDiscription, dbo.[tTimeTable2012-13].StaffID, dbo.[tTimeTable2012-13].DayID

FROM dbo.tSubject INNER JOIN

dbo.[tTimeTable2012-13] ON dbo.tSubject.SubjectID = dbo.[tTimeTable2012-13].SubjectID RIGHT OUTER JOIN

dbo.tPeriod ON dbo.[tTimeTable2012-13].PeriodID = dbo.tPeriod.PeriodID

WHERE ((dbo.[tTimeTable2012-13].StaffID = 18) AND (dbo.[tTimeTable2012-13].DayID = 1)) or

(dbo.[tTimeTable2012-13].StaffID is null)