Your profile information says you are running OSX 10.7.5, you say this is for 10.6.8 and you posted on the 10.5 forum. Answers may vary depending upon which computer software you are using.
Have you tried a different unzip utility such as The Unarchiver?
There may be something wrong with your Mac's automatic decompression but for now it is easy to use the Terminal:
1 - Open the Terminal application - in Utilities
2 - Type the following:
3 - type a space
4 - drag the file to be unzipped to the terminal window and release the mouse button
5 - hit the return key
6 - wait a while for a for the file to be unzipped and put into your Home folder