The island DOES appear on the worldmap. It shows up as a small island with a palm tree, like the ones showing a beach on the other islands. It does not show up as a "normal" island. But you can sail there just fine. As long as you have the compass, that is... <img src="style_emoticons/<#EMO_DIR#>/whistling.gif" style="vertical-align:middle" emoid="

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If you use <a href="http://www.s31clan.com/pietersmods/others/pb_modifications.zip" target="_blank">PB Modifications</a>, getting to Isla de Muerte is made much easier, because you can now get compass3 ingame as well: Go to the Tortuga tavern and talk to Mr. Gibbs who can be found there. He will give you the compass. You can also hire him as boatswain if you like.
The rarity and price settings for most of the new items, including the compasses, books and clocks are not balanced at all yet (remember: it IS still an Alpha version). If you find that they are too rare or too common, open up PROGRAM\ITEMS\initItems.c and change the values in that file. It is recommended you get Notepad++ from the Modding Tools page on my site. If you use that program to open the codefile, all columns will be lined up properly, making the file much more easy to edit. Be sure to tell me when you come up with better values than the current ones.
Morgan is right: If you have these items by default, there's no point in having them in the game at all. The idea with these items is to make you have to pay to gain some of the navigational features in the game. For example, needing a compass for the on-screen display and needing a chronometer and sextant to be able to see your own ship on the map interface. If you do not like this, you can turn off the item requirements using the following variables in BuildSettings.h: DISPLAY_SEA_TIME, DISPLAY_BATTLEINTERFACE, DISPLAY_MAP_POSITION.