download andy's upx from here or any other ready to use upx:

Then put it in your usr/bin/ folder.

Then use the command upx to pack pbs which is located in /System/Library/CoreServices.

The packing - compressing can be done with many commands , such as : upx -9 . which means compress better , or --ultra--brute , or --force e.t.c.

So in the terminal area type : upx -9 "here just drag the pbs binany on terminal" and hit enter.

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My Specs:

Motherboard: Tyan S2865 K8E
Cpu: Dual Core Amd Opteron 185 2600 MHz
Chipset: Nvidia Nforce4 chipset ultra
Graphics Card: Ati X700 Radeon Series
Lan Controller: Broadcom BCM 5721 GbE
Sound: Realtek ALC655 5.1 channel
Memory: DDR SDRAM PC3200 2048

System Version:
Mac OS X
10.6.3
10D575