Change it like this:
var stats:Object = {};
for(var i:Number=1; i<=90; i++) {
if(myLoader.data["st"+i]){
stats["st"+i] = myLoader.data["st"+i];
trace("st"+ i + " is: "+stats["st"+i]);
}
}
Note the syntax: myLoader.data["st"+i]
and not myLoader.data."st"+i