Type the word(s) that you want to search in the search box in the right upper part of the page and then you'll be sent to the search results page.
The search results page contains the results in a tabbed manner, each tab having the search results for a certain item type: tasks, milestones, messages, whiteboards and files. The number of results found is mentioned in the tab name.
The words that you searched for are highlighted in the results, for a better view.