Orchid,

There is an error in the DLookupo expression, in that you need ""s
around the first argument.  It also assumes that the ID field is Text
data type, is that correct?

Also, the Requery will not achieve anything, and can be removed.

Private Sub TerrCode_AfterUpdate()
  Me.empCode = DLookup("[Code]","DTNEmp","[ID]='" & Me![Emp] & "'")
End Sub

Having said that, there is a question about the validity of doing this
in the first place.  One of the fundamental rules of good database
design is that each value should be stored once only on one field in one
table.  It looks like you might be flouting this rule here.  The Code
corresponding with the ID is already defined within your database in the
other table, so if I am understanding correctly, all you really need
here is to *display* the value on the form, not to write it to a field.
 In that sense, what you are doing is very unusual.

There are a number of approaches that may be nore applicable.  This
article may help:
http://accesstips.datamanagementsolutions.biz/lookup.htm