you can use GoodZipRar to open rar file.
download URL:
GoodZipRaR for iPhone 3GS, iPhone 4, iPhone 4S, iPod touch (3rd generation), iPod touch (4th generation) and iPad on the iTunes App Store
► File Manager
- Folders support
- Rename, Copy, Move, Delete files
- Imports photo & video from Camera or Photos Library
- Supports photo thumbnails preview
- Supports Zip, Tar, RAR, CBR, CBZ
► Open Email Archive File
open files attached in emails, directly from your Mail, Open In...GoodZipRaR
►WiFi Share Batch Transfer
App will transform your Document and file handling forever. Its the first app that makes wireless transfer of your Document and files from or to your computer and other i-Devices. You need never use a cable again! 
- Batch transfer 
Just select, choose target and youre done. 
-No limitations 
One or thousands of Document or Movie from your last trip you can transfer them all at once.
- FILE SHARING:
File sharing with other iOS devices via USB or Wi-Fi