Try
FIND InvoiceHeader WHERE InvoiceHeader.InvoiceDate>=DATE(1,9,2011)
or just remove the ' as in
FIND InvoiceHeader WHERE InvoiceHeader.InvoiceDate>=01-09-2011
Your 4th option has a bracket missing after AS_DATE - that may be just a mistake in your post but would throw an error in the configuration tool
Finally, are you sure InvoiceHeader.InvoiceDate has it's format set as DD-MM-YYYY,
there are many possible causes but as Tom said, unless you post the error message it is about as much use as a chocolate coffee pot