With the advent of Mountain Lion, Apple installers need to be signed with a developer certificate. This is a precaution against malware. Unfortunately the installers made with those tools do not work on MacOS X 10.5 Leopard anymore. While our plug-ins do work on IntelMacs running 10.5, one has to manually install them. Here's how:
1. Download and install "Pacifist 3.0.10" from http://www.charlessoft.com/pacifist_olderversions.html - this is a tool that can extract contents from Apple installers.
2. If you haven't done so, download our installer, we'll use ACE for example.
3. Unpack ACE12Mac.zip and open the newly created folder ACE1560Mac
4. Right-Click (or Ctrl-Click) ACEInstaller.pkg and select "Open With->Pacifist.app"
(now Pacifist will ask you to register or wait a bit, it's a great software, you might need it more often on 10.5, so please consider buying it)
5. Authorize yourself by clicking the lock on the top right of the Pacifist window (see below)
6. Look into the Package contents and install the files accordingly:
Right-Click (Ctrl-Click) following files and choose "Install to Default Location"
ACE.vst3
ACE.vst
ACE.component
ACE
ACE.dpm
Images
Scripts
Here's a screenshot of what it looks like, and I've highlighted those files needed:

ACE is now installed!

Note: I have not verified this procedure on a Mac with MacOS X 10.5
#-------- one last thing ---------#
7. Final step, set permissions to usuable values:
- Right-Click following two folders in Finder and Get Info:
MacHD/Library/Audio/Presets/u-he
MacHD/Library/Application Support/u-he/ACE/Scripts
- Click the lock on the bottom right of the info windows, authorize as admin
- set permissions for everyone to read & write, so that you can switch UI and save presets!
- Apply permissions to the enclosed items
like here:

Let me know if/how it works...
- Urs
(I'm off to buy a Pacifist license now)