Need Help -- Migrating From Access To MS SQL

I am migrating my database from Access to MS SQL. I successfully migrated most of the Tables but I am going through a hellish time with queries/views

Can somwone tell me how to convert this query from Access to MS SQL:
-------------------------------------------------------------------------------------------------------------------
SELECT Manage_Pre_Award.A_ID, Manage_Pre_Award.Event_Name, Manage_Pre_Award.Plan_Date, Manage_Pre_Award.Actual_date, Manage_Pre_Award.Rev_Date, IIf([actual_date] Is Not Null,[actual_date],IIf([rev_date] Is Not Null,[rev_date],IIf([plan_date] Is Not Null,[plan_date],Null))) AS workdate,
FROM Manage_Pre_Award
WHERE (((Manage_Pre_Award.Rev_Date) Is Not Null)) OR (((Manage_Pre_Award.Actual_date) Is Not Null)) OR (((Manage_Pre_Award.Plan_Date) Is Not Null));

-----------------------------------------------------------------------------------------------------------------------

I have already tried this but get:
The Query Designer does not support the CASE SQL construct.

SELECT Manage_Pre_Award.A_ID, Manage_Pre_Award.Event_Name, Manage_Pre_Award.Plan_Date, Manage_Pre_Award.Actual_date,
Manage_Pre_Award.Rev_Date,
CASE WORKDATE
WHEN [actual_date] IS NOT NULL THEN [actual_date]
WHEN [actual_date] IS NULL AND [rev_date] IS NOT NULL THEN [rev_date] ] ELSE [plan_date]
END AS WORKDATE
FROM Manage_Pre_Award
WHERE (((Manage_Pre_Award.Rev_Date) IS NOT NULL)) OR
(((Manage_Pre_Award.Actual_date) IS NOT NULL))OR (((Manage_Pre_Award.Plan_Date) IS NOT NULL));