how i can open arj archive on my ipad?

You can't.

'ARJ is a file archiver that compresses files into a disk space saving archive. It is fast, reliable, and has a rich set of features which include Windows 9x, Millenium, 2000, XP, Vista, and 7 long filename support, file version management, data damage protection, configurable command options, multiple volume archives (archives that span removable media) and much more.'

From what I can see by Googling, ARJ archive is a windows-specific compression format. I can see no apps for opening them on iOS devices; this program may open them on a Mac:

http://www.downloadatoz.net/zipeg/