As far as I know must use VBA code to set the RowSource. Here is example from my project.
Code:
With Combo2
Select Case Combo1
Case ""
.Value = Null
.RowSourceType = ""
.RowSource = ""
Case "State Number"
.RowSourceType = "Table/Query"
.RowSource = "SELECT DISTINCT StateNum FROM Submit LEFT JOIN Projects ON Submit.ProjRecID=Projects.ProjRecID WHERE Not Projects.StateNum Is Null ORDER BY Projects.StateNum;"
Case "Sample Type"
.RowSourceType = "Value List"
.RowSource = "Acceptance;Independent Assurance;Information;Preconstruction;Quality"
Case "Material Type"
.RowSourceType = "Table/Query"
.RowSource = "SELECT MaterialType FROM MaterialType WHERE Active=True;"
Me.cbxTest.RowSource = "SELECT TestNum FROM MaterialTest WHERE MaterialType=[cbxFor" & Right(strSelection, 1) & "];"
Case "Program Code"
.RowSourceType = "Table/Query"
.RowSource = "SELECT ProgramCode FROM ProgramCodes;"
Case "Item Number"
.RowSourceType = "Table/Query"
.RowSource = "SELECT DISTINCT ItemNum FROM Submit;"
End Select
End With
Combo2.Requery