If using sql in VBA, the following would recognize the combo box:
SELECT [Exhibitor].[Exhibitor_Ref], [Exhibitor].[Exhibitor_Name], [Exhibitor].[Tel]
FROM Exhibitor INNER JOIN Booking ON [Exhibitor].[Exhibitor_Ref]=[Booking].[Exhibitor_Ref], Exhibitions
WHERE [Exhibitions].[Exhibition_Name] ='" & Forms!Form1!Combo1 & "' AND [Booking].[Paid] = 0;
If sql in a query, the following should recognize it:
SELECT [Exhibitor].[Exhibitor_Ref], [Exhibitor].[Exhibitor_Name], [Exhibitor].[Tel]
FROM Exhibitor INNER JOIN Booking ON [Exhibitor].[Exhibitor_Ref]=[Booking].[Exhibitor_Ref], Exhibitions
WHERE [Exhibitions].[Exhibition_Name] = Forms!Form1!Combo1 AND [Booking].[Paid] = 0;