I don't know about getting from an airport to Mello by public transport, but it must be possible as there are major towns in the main valley (Val Tellina).
There are 2 campsites in the village of San Martino, I stayed in the small one a little bit up the hill and it was great. A short walk into the village which has bars, restaurants and convenience store etc.
You can walk easily into the bouldering areas from the village, in fact vehicular access is restricted. One of the things that pissed me off during my visit is that it is obvious that this beautiful valley needs protection, yet climbers still insisted in camping and shitting in the woods to save on camp site fees. I found this hard to fathom to be honest.