For some strange reason, my solo privateer (I have one for running with a friend) cannot access the ancient tunnels. Oh, Samir could get in just fine when Vadima wanted him to go play tag with the idols, but can he get back in there to beat up crab crawlies for the diamonds? Nope! I run right up tot he door and smack into a wall. Not an issue when my husband can call up one of his pirates that is known to Samir. Then he goes into the tunnels and Samir can port to him. And then, assuming I don't get my body parts handed to me, I can fight the crab crawlies once or twice, maybe recover a single diamond, and then I have to leave to heal up (or recover a crew member) and the whole merry dance starts all over again. But it's not fair to constantly call for help (even if it is just across the room) when he wants to play his characters too.
What's up with that? My other pirates who have hit that point can go in and out of the tunnels at will. Samir's quest arrow doesn't even show the ancient tunnels when he's standing at the doorway!
Not sure why you are having this trouble and it would take some more information to find out. Like:
1) Did you complete the MooShu Falcon, Up for Crabs, The Jaaques of Diamonds, That's a Spicy Rat Haul, The Rat Pack Must Pay, Wake the Dead, or Black Magic Quests? If you didn't do them all, which ones do you have left? From what your Messages seems to indicate is that you haven't completed Up for Crabs yet.
2) What Quests are you currently working on?
3) What was the date and time that this occurred? KI modifies P101 frequently so hopefully one of the upgrades has fixed your problem.
Another possibility would depend on the type of account that you are using for that character. By type of account, I mean free, pay as you go, or membership (monthly or yearly).
The Quest Arrow only points to the Quest in your Quest Journal that you have highlighted (edges of the quest are white). If you don't have a Quest in the Ancient Tunnels, then the Quest Arrow can't point to it. It sounds like there is a problem with your character and/or account, that character has not completed the necessary quests, or that character is not a high enough level to get into the Ancient Tunnels
A way that I have used to get in and out of the Ancient Tunnels (or anywhere else) to heal myself is to make sure I am not in a dungeon (accessed by a Sigil) and in a safe location where I won't get into a fight. I then use the Skull icon in the lower left corner to transport me back to Avery's Court. This allows me to revive any defeated characters, heal myself, change what I have in my Backpack, my Bank, and my Shared Bank, and sell the stuff I don't want. When I am done, I use the Flame icon that the Skull icon became when I teleported to return to where I was (in this case the Ancient Tunnels). I believe you can go into the town of Skull Island and still use the Flame icon to return, just not much further. DO NOT log out or you will have to take the long way back to where your were.