To look for an exact phrase, use quotation marks (" ") around the keywords.
Example: "attention deficit disorder”
Note: this works in most search engines as well. If you type an exact phrase without quotations when doing a basic search, most search engines will look for each word separately. This means your result list will include web pages that not only contain the exact phrase (ex: attention deficit disorder) but also web pages that contain a word or words from the exact phrase appearing separately (ex: attention may appear in one paragraph or sentence and disorder will appear in another paragraph or sentence).
Look for the database's Help page. There is usually a link to the Help section of each database (e.g., Look for: Help, Tips, FAQ, Advanced Search). They usually provide good examples of how to search. You can always ask a librarian as well.