rs.CustName is errata on both counts. The line should read:
MsgBox "Customer name: " & rst!CustName
Bang (!) notation means that the identifier that follows refers to a member of a collection. CustName is a member of the Recordset's Fields collection. rst!CustName works because the Fields collection is the default property of a Recordset object (the Fields collection is implied).
Dot (.) notation means that the identifier that follows it refers to a property or method of the object. So since the Fields collection is a property of the Recordset object, you could also use: