I was having trouble with Antares Auto-Tune Evo, Auto-Tune EFX and QuikQuak’s Pitchwheel.
The fix is quite simple.
Go to the Digital Performer App in your Applications Folder. Right-Click (or Control+Click). Select “Get Info.” Check the box in the Get Info Window to “Open in 32-bit mode”
Now when DP starts, it will load those old plugins.
The reason is that DP8 is 64 bit. So 32-bit plugins are ignored. But DP 8 has a 32-bit mode for people like us who need to run old plugins.