I *believe* you simply need to do something like this:
Code:
UPDATE employeeTable
SET firstname = CASE When(employeeTable.firstName='N' and ID=10) THEN ValueTable.NewValue Else employeeTable.firstName End,
lastname = CASE When(employeeTable.lastname='N' and ID=20) THEN ValueTable.NewValue Else employeeTable.lastname End,
address = CASE When(employeeTable.address ='N' and ID=30) THEN ValueTable.NewValue Else employeeTable.address End
FROM employeeTable, ValueTable
where employeeTable.Empid = ValueTable.Empid
That is, you just need to repeat the table name in the FROM clause.