Don't understand what you mean, on Large land %, you often get one massive continent that goes all the way around the world. Is that what you mean by "not all islands"?
You don't need to edit the XML files to change the map generator. When you click 'New Game', where it says Land Size: Small/Medium/Large/Custom, set it to Custom. You get a new tab with a ton of settings to play with.
If you significantly bump up the very first number (Land masses take up %) then you're much less likely to get islands and much more likely to get one massive continent.
BTW on "Large" apparently the original MoM used 25% land but I upped it to 30% to avoid getting island-ish maps. The results I get on 30% seem to be more like the original MoM (which I always play on "Large") than when I had it at 25%.
Ummm, stupid question, you are actually running MoM IME aren't you, and not the original MoM? MoM IME isn't a mod - its a totally separate installed game - after its installed and you've set the location of your original LBX files, run the Server and then the Client from the shortcuts that get added to your start menu - don't run the original!