first of all,
cream1 is correct, with what was said, but i do not have much knowledge about mac's so i cannot help you if thats what you use.
If you are not using a Mac, then you can download winrar from
here winrar is just like winzip. If you have multiple files named like part1.rar part2.rar, then you will need to download all parts. Once you have all parts, simply double click on part1 and it will open up the rar file so you can drag out the attatchment.
When using winrar, you have a trial period, but once you trial is finished, winrar will still work, but you may get a small popup message to buy, which you can just simnply close.
hope this help's
regards
CP.