In Pokemon diamond I received vs seeker but not the dowsing machine how do get it?

You get the Dowsing Machine from Professor Rowan after you complete the Pokédex in Pokémon Diamond.

Here's a breakdown of the steps:

1. Complete the Pokédex: Catch or see all 150 Pokémon available in the Sinnoh region. You can find a complete list of Pokemon in the Sinnoh region on websites like Bulbapedia.

2. Return to Professor Rowan: Go back to Professor Rowan's laboratory in Sandgem Town.

3. Receive the Dowsing Machine: Professor Rowan will be happy with your dedication and will reward you with the Dowsing Machine.

Note: You can't get the Dowsing Machine before completing the Pokédex. If you have the VS Seeker but not the Dowsing Machine, you've likely not yet completed the Pokédex.