Why My index.php cannot be change?

i m using a 3 column theme call Xmark 1.0.2, now i can only add adsense at the sidebar widget but i would like to add adsense under the first post of my blog. So i open the index.php file, i replace my adsense code in this space:

<!–ADD YOUR YOUR ADSENSE CODE HERE–>

<!–replace this line with your code–>

<!–END ADSENSE–>

then i save the file. But when i reopen the file that i save just now, it shows: “Not Found
Sorry, but you are looking for something that isn’t here.”

What’s wrong?

Appreciate your help! TQ!