I'm running Magix Music Studio 15 (very much like Samplitude) and ran into this problem with the full version of Addictive Drums. It turns out Magix uses a VstPlugin.ini file, found in C:\Documents and Settings\All Users\Application Data\MAGIX\SamplitudeMusicStudio15. (Your path may vary depending on where you installed Magix.)
In VstPlugin.ini, I found the entry for "Addictive Drums" and changed it to the following. (Make sure Magix is NOT running at the time.)
"C:\Program Files\Steinberg\VstPlugins\Addictive Drums.dll",1,"Addictive Drums","",FFFFFFFF
Your plugin path may vary, but it's the part at the end that's important. After closing VstPlugin.ini and starting Magix, Addictive Drums showed up in the VSTi menu!
Note: I'm not sure why this change helps. I stumbled across it by examining files on another computer where AD was working properly. Hope it helps you too!