Copy the following code then press Alt+R in mIRC. This should open a popup window. Make sure you are on the tab labelled "Remotes". 
Go to File, New and paste the code onto the blank page. 
Replace ENTERAWAYNICKHERE with the nick you want to use when away and then hit ok. 
To go away type /aw your reason here and to come back type /bk



alias aw {
  set %away.msg $1-
  if ( $away == $true ) { echo -a You are already AWAY! | HALT }
  ame is now away: %away.msg Left at $time $+ .
  away %away.msg
  set %orig.nick $me
  set %away.time $ctime
  nick ENTERAWAYNICKHERE
}

alias bk {
  if ( $away == $false ) return
  ame is back from %away.msg at $time $+ . (Gone for $duration($calc(($ctime - %away.time))) $+ )
  away
  unset %away.msg
  nick %orig.nick
  unset %orig.nick
  unset %away.time
}