This is very strange indeed, especially if you have defeated all the Gym Leaders and earned all the Badges as you say, because your supposed to obtain the Move Tester app from the President of The Pokétch Company in Jubilife City when you defeat the Seventh Gym (Candice for the Icicle Badge).
A reason why the President of the Pokétch Company is not giving you the Move Tester app may be because you don't have all of the apps previous to the Move Tester. It's been a while since I played on Gen 4 but I'm pretty sure that you have to get the Pokétch apps in a particular order, so you can't get certain apps before others. To find out the numbered order of the Pokétch apps click on this link:
Note that even though the link above says Diamond and Pearl you can still use the numbered order for Platinum as the order in Platinum is identical to the order in Diamond and Pearl.
If you have all of the previous apps before the Move Tester then there may be a problem with your game and it could have a glitch. In any case I would try to fix it by using these methods in the numbered order:
- Soft Reset your Game (Hold Start, Select, LB and RB at the same time)
- Restart your Console
- Remove Game Cartridge and Give it a Good Clean
- Or as a Last Resort you could try Restarting your Game :(
Hope I helped :)