I don't think it works exactly as you're trying to use it. Often the subquery uses a different table trying to match some field value in your main query. Sometimes the subquery does some aggregation of the same table as the main query but uses an alias. I can't find an example of INSERT Into that uses the EXIST on the main table.
Have you got your INSERT sql without the Dual... organized? Can you post the SQL with the Exists as you tried it?
I would recommend that you use vba and DCount.
is perfectly valid, and is what you would use once you have determined that DCount = 0
sSQL = "INSERT INTO [Supervisor_Changes] ([Test Case ID],[Web Verifier Name],[Web Verifier Execution Date]) VALUES ('102-11 00030-E2E','Total_Power','4/28/2011 1:40:54 PM');"
I haven't seen your Update sql, but that's what you'd use if DCount = 1
You can try this in the immediate window.
?DCount("*","Supervisor_Changes","[Test Case ID]='102-11 00030-E2E'")
This should return 0 if the record doesn't exist (set up to do INSERT)
and 1 if the record does exist (set up to do UPDATE)