It would appear the problem is that you aren't using the same key name for the config and code.
Here's what you have in the config file:
<add key="DB" value="Provider=Microsoft.Jet.OLEDB.4.0;
Data Source=D:\My Documents\db.mdb;Jet OLEDB:Database Locking Mode=1;Mode=16" />
Here's what you have in the code:
_ConnectionString = ConfigurationManager.AppSettings["PlatinumDiamondDB"];
Those need to match.