What version of Action Replay Code Manager do you have? Right-click on the Top part of your Action Replay Code Manager then select the option: About Action Replay Code Manager... and it will tell you. Or try using Software upgrade so you get the latest version of Action Replay Code Manager. If software upgrade doesn't work, download the latest Action Replay Code Manager here: http://projectpokemon.org/forums/showthread.php?t=2783
Download the link depending on if you use Action Replay DS or Action Replay DS Media Edition.
I'll tell you a process of how to add Pokemon Platinum, if it doesn't work, it's probably because of the version of your Code Manager, if so do as you are told in the above details:
In your Code Manager, at the top-right corner is a box called Subscriptions. One of the subscriptions which should be there already is Codejunkies. Click Codejunkies US Games and scroll down until you see Pokemon Platinum. Click and drag Pokemon Platinum into your Codelist then add it to your connected DS. Or just drag it straight into the Nintendo DS box on the left side of the code manager.