First, there is no real difference between "Idle" or the "automatic Away". Even if you patch your Messenger, your contacts will still see you as "Away" unless they have patched their Messenger too.
The time before "Away" is automatically selected can be chosen, either in Messenger's own settings, or in MsgPlus's settings. Do
NOT use both at the same time!
To get the "automatically Away"-status, you can not use your computer at all, even the smallest movement on the mouse will change the status back to "Online". If you choose "Right back" or another status you will with Messenger's own settings never get "Idle", but with MsgPlus, you can set it to change to "automatic Away"/"Idle" even if you have chosen another status.
To have the "Idle"-status while using your computer, you need some software, MessengerDiscovery can do this - let you manually set your status to "Idle". Still, if your contacts have an unpatched Messenger, you are shown as "Away" in their windows, but in the MsgPlus's Event Log you will show up as inactive.
Hopefully this was a good description of it, English is not my native language, and sometimes I fell it a bit difficult to make myself clear...
/ –