Load the form that contains the field you wish to evaluate. I am not sure about your use of a text field and then limiting the input to a range of numeric values. this is problematic as numbers in a text field sort differently than numbers in a numerical field. eg in a text field, 002 sorts before 1. in a numeric field 002 sorts after one. this is important when you apply an IIF to your value.
Your selection of the after update event is correct. I am going on the assumption that you go to pure numerics to coreect your sort situation.
open a new macro in design view.
right click on its title bar and check "conditions" to make the conditions field visible. click in the conditions field, then enter "[MainDTCode] = 54" leave off the quotes when you type.
in the action field, enter "SetValue" down below there will be an "item" textbox and an "expression" text box in the item text box enter [Code] and in the expression box put "CHG"
you can also enter ranges in the conditions field by using ([MainDTCode] < 99) And ([MainDTCode] > 50)
you may also choose to use the fully qualified names of the fields to reduce ambiguity eg. "[Forms]![Member Types]![MemberTypeID]"
you can also do this in VB but a macro will cause you fewer headaches.
please rate my answer.