I use Centos 5 and plesk 8.4.0, and i made:
1) wget http://3es.atomicrocketturtle.com/installers/atomic.sh
2) sh atomic.sh
3) nano /etc/yum.repos.d/CentOS-Base.repo
4)
[atomic]
name=Atomic Rocket Turtle – $releasever – Atomic PSA-Compatible RPMS
baseurl=http://www.atomicorp.com/channels/atomic/centos/$releasever/$basearch
[plesk]
name=Plesk Server Administrator
baseurl=http://www.atomicorp.com/channels/plesk/latest/centos/$releasever/$basearch
gpgcheck=0
5) yum update
6) restart
At plesk webadmin, plesk version is psa v8.4.0_build84080514.18 os_CentOS 5