I believe you need to specify a distinct value for the criterion (the last field) rather than just point at a field. For instance, to retrieve the [Description] from "Plan Codes" where [Plan Code] is equal to, say, "Primary" and assign it to the string variable strDesc, you'd use something like:
Code:
strDesc = DLookup("[Description]","Plan Codes","[Pay Plan]= 'Primary'")
If your [Pay Plan] value is displayed in a text box named txtPayPlan, you'd use:
Code:
strDesc = DLookup("[Description]","Plan Codes","[Pay Plan]= '" & txtPayPlan.Value & "'")