Lookup uses a relationship to pick the matching record, so you have to work on the relationship's definition if you want a different record. In other words you have to make sure the relationship itself goes to the record you want.
If you can't do that, because you want the most recent value but you don't know what the date of the most recent entry is and therefore can't use it in the relationship's definition, a possible alternative is to use ExecuteSQL, but that might turn out being slow:
lkValues = ExecuteSQL("SELECT yourValues FROM yourTable WHERE yourMatch = ? ORDER by yourDateField DESC";"";""; matchField);
Modify your relationship to match on both pairs of fields.
If you can't change the current relationship without affecting other parts of your solution, create a new occurrence of this look up table and use it for this relationship.
Sort your relationship by date in descending order. You can go to the relationship graph, double click the relationship line and get a dialog where you can specify a sort order.