Use the ISNULL function:
Select mp.MPCode, convert(varchar(102), mp.MaintenceStartDate, 106) as MaintenceStartDate,
mp.Description, mp.MaintencePlanType, mp.MDCode, ISNULL(md.MachinaryName, '???') + '-' + ISNULL(md.Model, '???') as [Machine Name and Model]
from tblMaintencePlanEntry mp inner join tblMachinaryDetails md on mp.MDCode=md.MDCode order by mp.MPCode desc