Try a program called MacPAR deLuxe. Its primary function is to create par2 files, though I haven't yet made use of that function. I mainly use it to extract from RAR files on my Mac, and it has never let me down. Make sure you get the right version for your system: the newest is for Snow Leopard only, but there is one for Tiger and Leopard and another for Panther 10.3.9.

The 5.1 was RAR'd by Satanika and they should both be a little under 200 mb. The extracted file is 388.1 mb (406,970,368 bytes).