obviously the Akai manual isn't going to tell you how to restore your OS after you install a third-party, unofficial, unsupported OS
but the instructions are on the JJOS site, which you read before you installed JJOS, right?
you did read them, right?
you saw the bit about holding down the magic key combination so that you don't permanently install JJOS and make it impossible to reinstall the Akai OS, right?
just kidding. but read the last line of this page:
http://www7a.biglobe.ne.jp/~mpc1000/update312.htmyou download the akai OS from akaipro.com and follow those instructions