I am using the twenty-eleven theme and every page (except the 404) has a sidebar. However the post permalink page (/2011/11/post-name/) does not. How do i put the sidebar onto it? I tried looking for its php file on codex but it is very unclear which file is the correct one.
The template for the display of a single post is
Note: This will get overwritten, if you ever update the theme.
Without looking in the theme folder for Twenty-Eleven, my guess is you need to alter the template file titled "content-link.php"...or whichever one that theme uses to display that page.
Then either create a custom sidebar template, or simply link the existing one itself by using:
Sorry I misinterpreted what you meant in your title...
Try this in your single.php template instead -