Type in the Init Box of the unit you want to sit:
[this] Exec "SitDown.sqs"
Then create a script file called SitDown.sqs (with notepad or such program) and type:
_Unit = _This Select 0
_Unit SwitchMove "EffectStandSitDown"
Then save the SitDown.sqs file in your mission folder.
You can also write in the place of "EffectStandSitDown", "EffectStandSitDownVer1" or "EffectStandSitDownVer1"
hope this helps You