Hi & Welcome to the Forum,not a Mac user myself so not really able to offer much real assistance
On the PC you "generally" have to extract the file (can be broken into more then 1 part so you do need all parts)....before you can view it . For the PC there are some shareware/freeware progs (7-Zip)which allow you to extract the files altho I use the official Winrar.
Can only suggest......
Does the Mac have a knowledge base like Windows does? Try searching for Rar files there or failing that Google or a Mac Forum (not knowledgeable about macs to recommend a Forum link)
Have you checked out the Winrar Home page
http://www.rarlab.com/
Sorry not much help,hopefully some other members who use Macs maybe of help
Hope you get it sorted out ,good luck
PS apologies if youve tried these suggestions already