If it was number (2) which uses と then the meaning before it would have to be 'whenever something happens' or 'if that happens'. But the section after と has then got to be something definite, you can't put a request after it. It's got to be 'if this, then this'.
So, just like you can't say something like 'whenever I look out the window, can you get my socks please' Although that almost makes sense in English... if your grammar is just a bit out
So the answer is the first one:
Can I just add something that I found interesting about this sentence. At first I didn't get the 2nd one's meaning. I translated 教えて as 'teach' - as you often do.
But for this particular sentence that's not the right shade of meaning. 'If you arrive, please teach me" - teach me what?
But of course, in this context, it's more like 'tell'. So it's:
If you arrive please tell me or
If and when you arrive, please let me know or even
When you arrive, let me know
Because たら can also refer to something inevitable, not just conditional. See Minna No Nihongo, Grammar Notes guide on that one.