I'm having problems installing .sf2 files HELP!

I'm using logic pro 8, and have download various softsynths in the past without any difficulty. I've followed the advice given on various websites and downloaded sfark and sfpack to decompress the files, however none of the files i download are able to 'unzip,' and simply tell me the 'decompression failed' as soon as the download has finished.

i really need a step-by-step guide of getting the sf2 from the website to logic, because it's doing my nut.

Thanks