Again, try DEBUGGING!
I used Chrome and its debugger and immediately got
Which is correct. It needs to be
var currentButton = getElementById(choice);
Uncaught ReferenceError: getElementById is not defined
But that's not the only problem. You have some nasty logic errors in the code. Clicking on the wrong button ("miss") can cause it to start showing TWO boxes colored, and worse. HINT: You aren't clearing the timeout upon a miss!!! (But there may be other bugs.)
Oh, and this is UTTERLY BOGUS (the part in red):
document.write('<td width=10%><form><input type="button" value="X" id=' + (y*10+x) + ' this.style.backgroundColor=\'white\' onclick="point()"></form></td>')