This is the script I use to take the user ID and write out their name to a UDF. You should be able to see how to use this to do what you want.

sFirst = "" : sLast = "" : sKey= "" : sCKey = ""

retVal = oBusObj.GetValue("UserCreatedKey", sCKey)

Set oUser = oBusObj.AsObject(oBusObj.GetChildHandle("UserCreatedKey"))

retVal = oUser.getvalue("UserKey$",sKey)
retVal = oUser.getvalue("FirstName$",sFirst)
retVal = oUser.getvalue("LastName$",sLast)

If sKey = sCKey then
 retVal = oBusObj.SetValue("UDF_FIRST$", sFirst)
 retVal = oBusObj.SetValue("UDF_LAST$", sLast)

end if

I don't need the username of the created key. I want the user key (it could be user code as well) that printed the order. I need the user who changed the PickingSheetPrinted from Y to N whether through Picking Sheet Printing or through Quick Print. My understanding is the oSession.UserKey should get me the user key of the session user. However I tried using post validate on the PickingSheetPrinted as well as post Table Write and the script seems to do nothing. 

I don't need the username of the created key. I want the user key (it could be user code as well) that printed the order. I need the user who changed the PickingSheetPrinted from Y to N whether through Picking Sheet Printing or through Quick Print. My understanding is the oSession.UserKey should get me the user key of the session user. However I tried using post validate on the PickingSheetPrinted as well as post Table Write and the script seems to do nothing.