I'm not sure why the More tag isn't working, but this should do what you want -
Edit the content.php file in the theme folder and look for the following line (near line 55):
<?php if ( is_archive() || is_search() ) : // Only display excerpts for archives and search. ?>
Change it to:
<?php if ( is_archive() || is_search() || !is_singular() ) : // Display excerpts for archive, search, and any area that is not a single post or page. ?>
Hopefully that helps!