Because of a bug in the installer for Zero-G's World Pack, there may be some additional steps you have to take to install the product correctly. If you keep getting error messages when trying to install, try one of the methods below. If you have an external drive, you can use the External HD method. If not, use the Single HD method.

Single HD

You can use Disk Utility to make a blank disk image.

Open Disk Utility, located in Applications/Utilities, don't select anything in the Disk Utility window.
Choose File > New > Blank Image from the menu
Type a name for the disk image and choose where you want to save it, i.e. WorldPack on the Desktop.
Set the size of the disk image from the Size pop-up menu to Custom 2gb.
Leave encryption method set to none.
Leave format set to read/write disk image, then click Create.

Now run the installer (Install Zero-G WorldPack Instruments.mpkg) from the DVD and select WorldPack as the destination.

External HD

Run the installer and select the external HD.

How to copy the files over to your System

In the finder, select WorldPack or your external disc and drag the contents of this directory:

Library / Application Support / GarageBand / Instrument Library
into this one:

Your HD / Users / Your User Name / Library / Application Support / GarageBand / Instrument Library

Then, restart your Mac and Trash the WorldPack.dmg on your Desktop (or the Library folder on your external HD). GarageBand will now see the WorldPack Instruments.