When you select a session you can view the log messages associated with that session on the Log tab.
You can find an arbitrary block of text in any of the main string fields of the log - caption, description, details, exception message, or call stack - by entering the text in the search box and pressing enter. It isn't necessary (or desirable) to put any extra search characters around the text. Search is performed case and culture insensitive.
On each log message you can click the Show Other Messages Like This button. This uses the same algorithm that merges warnings & errors together to create events but can be used on any message of any severity. When filtered this way a message is displayed in the navigation box to remove this filter.
When the log isn't being displayed with all messages visible in timeline order you can select the option to view just the messages leading up to the selected message. This displays a new view of the messages filtered to just show the last 50 messages logged within a few minutes and to the same thread or user as the selected message. Even on busy server applications this will show the likely messages that lead up to the one you've selected.
This is particularly handy to pivot from an error to the true error that happened first, causing the downstream problem.
For sessions that contain data related to multiple users (typically web applications) you can restrict the view to just a single user's activity by finding any message for that user, clicking on their name and selecting Filter to User. You can also click the Filter button and select the user from the list of users in the session.
You can also navigate to the log from the Users tab on the Session to see the log filtered by the user.