It's an expression, so it will need to be used in something like "compare two values" or "is between values" which are system conditions.
loopindex will reference the current loop being run, so if it's a sub event to your loop (or used as an additional condition or action expression) then it will not need any reference to which loop.
In your condition simply go to System -> compare two values and in the top box just type in loopindex and then set "is equal" and in the bottom box put 48 or 49 depending on your loop starting position.
~Sol