Usage:
This will show you this page
!define GAME "counter-strike source"
!include steamUserSelect.nsh
!insertmacro SteamUserSearch

After the page has completed you can access thease Vars
$steamPath == Path To Steam Installation Dir
$steamUser == Selected Steam User
$steamUsers == Int Containing #Accounts With The Selected GAME
So if you use !define GAME "counter-strike source", than the path to the game will be
"$steamPath\SteamApps\$steamUser\${GAME}\"